Light-emitting element, display device, electronic device, and lighting device

ABSTRACT

A light-emitting element containing a fluorescent material and having high emission efficiency is provided. The light-emitting element contains the fluorescent material and a host material. The host material contains a first organic compound and a second organic compound. The first organic compound and the second organic compound can form an exciplex. The minimum value of a distance between centroids of the fluorescent material and at least one of the first organic compound and the second organic compound is 0.7 nm or more and 5 nm or less.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a light-emitting element, or a display device, an electronic device, and a lighting device each including the light-emitting element.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a storage device, a method of driving any of them, and a method of manufacturing any of them.

2. Description of the Related Art

In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By application of a voltage between the electrodes of this element, light emission from the light-emitting substance can be obtained.

Since the above light-emitting element is a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Furthermore, such a light-emitting element also has advantages in that the element can be manufactured to be thin and lightweight, and has high response speed.

In a light-emitting element whose EL layer contains an organic compound as a light-emitting substance and is provided between a pair of electrodes (e.g., an organic EL element), application of a voltage between the pair of electrodes causes injection of electrons from a cathode and holes from an anode into the EL layer having a light-emitting property and thus a current flows. By recombination of the injected electrons and holes, the light-emitting organic compound is brought into an excited state to provide light emission.

Note that an excited state formed by an organic compound can be a singlet excited state (S*) or a triplet excited state (T*). Light emission from the singlet-excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The formation ratio of S* to T* in the light-emitting element is 1:3. In other words, a light-emitting element containing a compound emitting phosphorescence (phosphorescent compound) has higher emission efficiency than a light-emitting element containing a compound emitting fluorescence (fluorescent compound). Therefore, light-emitting elements containing phosphorescent compounds capable of converting a triplet excited state into light emission has been actively developed in recent years.

Among light-emitting elements containing phosphorescent compounds, a light-emitting element that emits blue light in particular has yet been put into practical use because it is difficult to develop a stable compound having a high triplet excited energy level. For this reason, the development of a light-emitting element containing a more stable fluorescent compound has been conducted and a technique for increasing the emission efficiency of a light-emitting element containing a fluorescent compound (fluorescent element) has been searched.

As one of materials capable of partly converting the triplet excited state into light emission, a thermally activated delayed fluorescent (TADF) emitter has been known. In a thermally activated delayed fluorescent emitter, a singlet excited state is generated from a triplet excited state by reverse intersystem crossing, and the singlet excited state is converted into light emission.

In order to increase emission efficiency of a light-emitting element using a thermally activated delayed fluorescent emitter, not only efficient generation of a singlet excited state from a triplet excited state but also efficient emission from a singlet excited state, that is, high fluorescence quantum yield are important in a thermally activated delayed fluorescent emitter. It is, however, difficult to design a light-emitting material that meets these two.

Patent Document 1 discloses a method: in a light-emitting element containing a thermally activated delayed fluorescent emitter and a fluorescent compound, singlet excitation energy of the thermally activated delayed fluorescent emitter is transferred to the fluorescent compound and light emission is obtained from the fluorescent compound.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2014-45179

SUMMARY OF THE INVENTION

In order to increase emission efficiency of a light-emitting element containing a thermally activated delayed fluorescent emitter and a fluorescent compound, efficient generation of a singlet excited state from a triplet excited state is preferable. In addition, efficient energy transfer from a singlet excited state of the thermally activated delayed fluorescent emitter to a singlet excited state of the fluorescent compound is preferable. Moreover, energy transfer from a triplet excited state of the thermally activated delayed fluorescent emitter to a triplet excited state of the fluorescent compound is preferably inhibited.

In view of the above, an object of one embodiment of the present invention is to provide a light-emitting element that contains a fluorescent compound and has high emission efficiency. Another object of one embodiment of the present invention is to provide a light-emitting element with low power consumption. Another object of one embodiment of the present invention is to provide a novel light-emitting element. Another object of one embodiment of the present invention is to provide a novel light-emitting device. Another object of one embodiment of the present invention is to provide a novel display device.

Note that the description of the above object does not preclude the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like.

One embodiment of the present invention is a light-emitting element including an EL layer. The EL layer includes a light-emitting layer in which an exciplex is formed, whereby triplet excitons can be converted into singlet excitons and light can be emitted from the singlet excitons. The light-emitting element can emit light from a fluorescent compound by utilizing energy transfer of the singlet excitons.

Thus, one embodiment of the present invention is a light-emitting element including a fluorescent material and a host material. The host material contains a first organic compound and a second organic compound. The first organic compound and the second organic compound can form an exciplex. The minimum value of a distance between centroids of the fluorescent material and at least one of the first organic compound and the second organic compound is 0.7 nm or more and 5 nm or less.

Another embodiment of the present invention is a light-emitting element including a fluorescent material and a host material. The fluorescent material has a fluorescence quantum yield of 70% or higher. The host material contains a first organic compound and a second organic compound. The first organic compound and the second organic compound can form an exciplex. The minimum value of a distance between centroids of the fluorescent material and at least one of the first organic compound and the second organic compound is 0.7 nm or more and 5 nm or less.

In each of the above structures, the minimum value of the distance between the centroids of the first organic compound and the fluorescent material and the minimum value of the distance between the centroids of the second organic compound and the fluorescent material are each preferably 0.7 nm or more and 5 nm or less. Further, the minimum value of the distance between the centroids is preferably a distance at the time when the running coordination number calculated by a classical molecular dynamics method exceeds 0.

In each of the above structures, the fluorescent material preferably includes at least two alkyl groups each having 2 or more carbon atoms. Alternatively, the fluorescent material preferably includes at least two branched alkyl groups each having 3 to 10 carbon atoms. Alternatively, the fluorescent material preferably includes at least two cyclic hydrocarbon groups each having 3 to 10 carbon atoms or at least two bridged cyclic hydrocarbon groups each having 3 to 10 carbon atoms.

In each of the above structures, the fluorescent material preferably includes condensed aromatic hydrocarbon having 3 to 12 carbon atoms.

In each of the above structures, the exciplex preferably gives excited energy to the fluorescent material.

In each of the above structures, light emitted from the exciplex preferably has a region overlapping with an absorption band of the fluorescent material on the lowest energy side.

In each of the above structures, the proportion of a delayed fluorescence component in light emitted from the exciplex is preferably higher than or equal to 10%, and the delayed fluorescence component preferably contains a delayed fluorescence component whose fluorescence lifetime is 10 ns or longer and 50 μs or shorter.

In each of the above structures, one of the first organic compound and the second organic compound preferably has a function of transporting electrons, and the other of the first organic compound and the second organic compound preferably has a function of transporting holes. Alternatively, one of the first organic compound and the second organic compound preferably includes a π-electron deficient heteroaromatic ring skeleton, and the other of the first organic compound and the second organic compound preferably includes a π-electron rich heteroaromatic ring skeleton or an aromatic amine skeleton.

Another embodiment of the present invention is a display device including the light-emitting element having any of the above-described structures, and at least one of a color filter and a transistor. Another embodiment of the present invention is an electronic device including the display device, and at least one of a housing and a touch sensor. Another embodiment of the present invention is a lighting device including the light-emitting element having any of the above-described structures, and at least one of a housing and a touch sensor. The category of one embodiment of the present invention includes not only the light-emitting device including the light-emitting element but also an electronic device including the light-emitting device. Accordingly, the light-emitting device in this specification refers to an image display device and a light source (e.g., a lighting device). The light-emitting device may be included in a display module in which a connector such as a flexible printed circuit (FPC) or a tape carrier package (TCP) is connected to a light-emitting device, a display module in which a printed wiring board is provided on the tip of a TCP, or a display module in which an integrated circuit (IC) is directly mounted on a light-emitting element by a chip on glass (COG) method.

According to one embodiment of the present invention, a light-emitting element that has high emission efficiency and contains a fluorescent compound can be provided. According to another embodiment of the present invention, a light-emitting element with low power consumption can be provided. According to another embodiment of the present invention, a novel light-emitting element can be provided. According to another embodiment of the present invention, a novel light-emitting device can be provided. According to another embodiment of the present invention, a novel display device can be provided.

Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily have all the effects described above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B are schematic cross-sectional views of a light-emitting element of one embodiment of the present invention and FIG. 1C is a diagram illustrating the correlation of energy levels in a light-emitting layer;

FIG. 2 illustrates a positional relationship between a host material and a guest material;

FIG. 3 illustrates the running coordination number of a host material around a guest material;

FIGS. 4A and 4B each illustrate a positional relationship between a host material and a guest material;

FIGS. 5A to 5C each illustrate the most stable structure of a host material and a guest material in a triplet excited state;

FIGS. 6A and 6B are schematic cross-sectional views of a light-emitting element of one embodiment of the present invention and FIG. 6C is a diagram illustrating the correlation of energy levels in a light-emitting layer;

FIGS. 7A and 7B are schematic cross-sectional views of a light-emitting element of one embodiment of the present invention and FIG. 7C is a diagram illustrating the correlation of energy levels in a light-emitting layer;

FIGS. 8A and 8B are each a schematic cross-sectional view of a light-emitting element of one embodiment of the present invention;

FIGS. 9A and 9B are each a schematic cross-sectional view of a light-emitting element of one embodiment of the present invention;

FIGS. 10A to 10C are schematic cross-sectional views illustrating a method for manufacturing a light-emitting element of one embodiment of the present invention;

FIGS. 11A to 11C are schematic cross-sectional views illustrating a method for manufacturing a light-emitting element of one embodiment of the present invention;

FIGS. 12A and 12B are a top view and a schematic cross-sectional view illustrating a display device of one embodiment of the present invention;

FIGS. 13A and 13B are schematic cross-sectional views each illustrating a display device of one embodiment of the present invention;

FIG. 14 is a schematic cross-sectional view illustrating a display device of one embodiment of the present invention;

FIGS. 15A and 15B are schematic cross-sectional views each illustrating a display device of one embodiment of the present invention;

FIGS. 16A and 16B are schematic cross-sectional views each illustrating a display device of one embodiment of the present invention;

FIG. 17 is a schematic cross-sectional view illustrating a display device of one embodiment of the present invention;

FIGS. 18A and 18B are schematic cross-sectional views each illustrating a display device of one embodiment of the present invention;

FIGS. 19A and 19B are a block diagram and a circuit diagram illustrating a display device of one embodiment of the present invention;

FIGS. 20A and 20B are circuit diagrams each illustrating a pixel circuit of a display device of one embodiment of the present invention;

FIGS. 21A and 21B are circuit diagrams each illustrating a pixel circuit of a display device of one embodiment of the present invention;

FIGS. 22A and 22B are perspective views of an example of a touch panel of one embodiment of the present invention;

FIGS. 23A to 23C are cross-sectional views of examples of a display device and a touch sensor of one embodiment of the present invention;

FIGS. 24A and 24B are cross-sectional views each illustrating an example of a touch panel of one embodiment of the present invention;

FIGS. 25A and 25B are a block diagram and a timing chart of a touch sensor of one embodiment of the present invention;

FIG. 26 is a circuit diagram of a touch sensor of one embodiment of the present invention;

FIG. 27 is a perspective view of a display module of one embodiment of the present invention;

FIGS. 28A to 28G illustrate electronic devices of one embodiment of the present invention;

FIGS. 29A to 29C are a perspective view and cross-sectional views illustrating a light-emitting device of one embodiment of the present invention;

FIGS. 30A to 30D are cross-sectional views each illustrating a light-emitting device of one embodiment of the present invention;

FIGS. 31A to 31C illustrate a lighting device and an electronic device of one embodiment of the present invention;

FIG. 32 illustrates lighting devices of one embodiment of the present invention;

FIG. 33 shows current efficiency-luminance characteristics of light-emitting elements in Example;

FIG. 34 shows external quantum efficiency-luminance characteristics of light-emitting elements in Example;

FIG. 35 shows luminance-voltage characteristics of light-emitting elements in Example;

FIG. 36 shows electroluminescence spectra of light-emitting elements in Example;

FIGS. 37A and 37B show fluorescence lifetime characteristics of light-emitting elements in Example;

FIGS. 38A and 38B show transient electroluminescence spectra of light-emitting elements in Example;

FIGS. 39A and 39B show transient electroluminescence spectra of light-emitting elements in Example;

FIG. 40 shows current efficiency-luminance characteristics of light-emitting elements in Example;

FIG. 41 shows external quantum efficiency-luminance characteristics of light-emitting elements in Example;

FIG. 42 shows luminance-voltage characteristics of light-emitting elements in Example;

FIG. 43 shows electroluminescence spectra of light-emitting elements in Example;

FIGS. 44A and 44B show results of time-resolved fluorescence measurement of light-emitting elements in Example;

FIGS. 45A and 45B show transient electroluminescence spectra of light-emitting elements in Example;

FIGS. 46A and 46B show transient electroluminescence spectra of light-emitting elements in Example;

FIG. 47 shows current efficiency-luminance characteristics of light-emitting elements in Example;

FIG. 48 shows external quantum efficiency-luminance characteristics of light-emitting elements in Example;

FIG. 49 shows luminance-voltage characteristics of light-emitting elements in Example;

FIG. 50 shows electroluminescence spectra of light-emitting elements in Example;

FIG. 51 shows results of time-resolved fluorescence measurement of thin films in Example; and

FIGS. 52A and 52B show results of time-resolved fluorescence measurement of thin films in Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to description to be given below, and it is to be easily understood that modes and details thereof can be variously modified without departing from the purpose and the scope of the present invention. Accordingly, the present invention should not be interpreted as being limited to the content of the embodiments below.

Note that the position, the size, the range, or the like of each structure illustrated in drawings and the like is not accurately represented in some cases for simplification. Therefore, the disclosed invention is not necessarily limited to the position, the size, the range, or the like disclosed in the drawings and the like.

Note that the ordinal numbers such as “first”, “second”, and the like in this specification and the like are used for convenience and do not denote the order of steps or the stacking order of layers. Therefore, for example, description can be made even when “first” is replaced with “second” or “third”, as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as those which specify one embodiment of the present invention.

In the description of modes of the present invention in this specification and the like with reference to the drawings, the same components in different diagrams are commonly denoted by the same reference numeral in some cases.

In this specification and the like, the terms “film” and “layer” can be interchanged with each other depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.

In this specification and the like, a singlet excited state (S*) refers to a singlet state having excited energy. The lowest level of the singlet excited energy level (S1 level) refers to the excited energy level of the lowest singlet excited state. A triplet excited state (T*) refers to a triplet state having excited energy. The lowest level of the triplet excited energy level (T1 level) refers to the excited energy level of the lowest triplet excited state.

In this specification and the like, a fluorescent material or a fluorescent compound refers to a material or a compound that emits light in the visible light region when the relaxation from the singlet excited state to the ground state occurs. A phosphorescent material or a phosphorescent compound refers to a material or a compound that emits light in the visible light region at room temperature when the relaxation from the triplet excited state to the ground state occurs. That is, a phosphorescent material or a phosphorescent compound refers to a material or a compound that can convert triplet excited energy into visible light.

Note that in this specification and the like, “room temperature” refers to a temperature higher than or equal to 0° C. and lower than or equal to 40° C.

In this specification and the like, a wavelength range of blue refers to a wavelength range of greater than or equal to 400 nm and less than 490 nm, and blue light has at least one peak in that wavelength range in an emission spectrum. A wavelength range of green refers to a wavelength range of greater than or equal to 490 nm and less than 580 nm, and green light has at least one peak in that wavelength range in an emission spectrum. A wavelength range of red refers to a wavelength range of greater than or equal to 580 nm and less than or equal to 680 nm, and red light has at least one peak in that wavelength range in an emission spectrum.

(Embodiment 1)

In this embodiment, a light-emitting element of one embodiment of the present invention will be described below with reference to FIGS. 1A to 1C, FIG. 2, FIG. 3, FIGS. 4A and 4B, and FIGS. 5A to 5C.

<Structure Example of Light-emitting Element>

First, a structure of the light-emitting element of one embodiment of the present invention will be described with reference to FIGS. 1A to 1C.

FIG. 1A is a schematic cross-sectional view of a light-emitting element 250 of one embodiment of the present invention.

The light-emitting element 250 includes a pair of electrodes (an electrode 101 and an electrode 102) and an EL layer 100 between the pair of electrodes. The EL layer 100 includes at least a light-emitting layer 130.

The EL layer 100 illustrated in FIG. 1A includes functional layers such as a hole-injection layer 111, a hole-transport layer 112, an electron-transport layer 118, and an electron-injection layer 119, in addition to the light-emitting layer 130.

In this embodiment, although description is given assuming that the electrode 101 and the electrode 102 of the pair of electrodes serve as an anode and a cathode, respectively, they are not limited thereto for the structure of the light-emitting element 250. That is, the electrode 101 may be a cathode, the electrode 102 may be an anode, and the stacking order of the layers between the electrodes may be reversed. In other words, the hole-injection layer 111, the hole-transport layer 112, the light-emitting layer 130, the electron-transport layer 118, and the electron-injection layer 119 may be stacked in this order from the anode side.

The structure of the EL layer 100 is not limited to the structure illustrated in FIG. 1A, and a structure including at least one layer selected from the hole-injection layer 111, the hole-transport layer 112, the electron-transport layer 118, and the electron-injection layer 119 may be employed. Alternatively, the EL layer 100 may include a functional layer which is capable of lowering a hole- or electron-injection barrier, improving a hole- or electron-transport property, inhibiting a hole- or electron-transport property, or suppressing a quenching phenomenon by an electrode, for example. Note that the functional layers may each be a single layer or stacked layers.

FIG. 1B is a schematic cross-sectional view illustrating an example of the light-emitting layer 130 in FIG. 1A. The light-emitting layer 130 in FIG. 1B includes a host material 131 and a guest material 132. The host material 131 includes an organic compound 131_1 and an organic compound 131_2.

The guest material 132 may be a light-emitting organic compound, and the light-emitting organic compound is preferably a substance capable of emitting fluorescence (hereinafter also referred to as a fluorescent compound). A structure in which a fluorescent compound is used as the guest material 132 will be described below. The guest material 132 may be referred to as the fluorescent material or the fluorescent compound.

In the light-emitting element 250 of one embodiment of the present invention, voltage application between the pair of electrodes (the electrodes 101 and 102) allows electrons and holes to be injected from the cathode and the anode, respectively, into the EL layer 100 and thus a current flows. By recombination of the injected carriers (electrons and holes), excitons are formed. The ratio of singlet excitons to triplet excitons (hereinafter referred to as exciton generation probability) which are generated by carrier (electrons and holes) recombination is approximately 1:3 according to the statistically obtained probability. Accordingly, in a light-emitting element that uses a fluorescent material, the probability of generation of singlet excitons, which contribute to light emission, is 25% and the probability of generation of triplet excitons, which do not contribute to light emission, is 75%. Therefore, converting the triplet excitons, which do not contribute to light emission, into singlet excitons, which contribute to light emission, is important in increasing the emission efficiency of the light-emitting element.

<Light Emission Mechanism of Light-emitting Element>

Next, the light emission mechanism of the light-emitting layer 130 is described below.

The organic compound 131_1 and the organic compound 131_2 included in the host material 131 in the light-emitting layer 130 form an exciplex.

Although it is acceptable as long as the combination of the organic compound 131_1 and the organic compound 131_2 can form an exciplex, it is preferable that one of them be a compound having a function of transporting holes (a hole-transport property) and the other be a compound having a function of transporting electrons (an electron-transport property). In that case, a donor-acceptor exciplex is formed easily; thus, efficient formation of an exciplex is possible. In the case where the combination of the organic compounds 131_1 and 131_2 is a combination of a compound having a hole-transport property and a compound having an electron-transport property, the carrier balance can be easily controlled depending on the mixture ratio. Specifically, the weight ratio of the compound having a hole-transport property to the compound having an electron-transport property is preferably within a range of 1:9 to 9:1. Since the carrier balance can be easily controlled with the structure, a carrier recombination region can also be controlled easily.

In order to efficiently form an exciplex, the combination of the host materials preferably satisfies the follows: the highest occupied molecular orbital (also referred to as HOMO) level of one of the organic compound 131_1 and the organic compound 131_2 is higher than the HOMO level of the other of the organic compounds, and the lowest unoccupied molecular orbital (also referred to as LUMO) level of the one of the organic compounds is higher than the LUMO level of the other of the organic compounds. For example, when one of the organic compounds has a hole-transport property and the other of the organic compounds has an electron-transport property, it is preferable that the HOMO level of the one of the organic compounds be higher than the HOMO level of the other of the organic compounds and the LUMO level of the one of the organic compounds be higher than the LUMO level of the other of the organic compounds. Specifically, a difference in HOMO level between the organic compounds is preferably greater than or equal to 0.05 eV, further preferably greater than or equal to 0.1 eV, and still further preferably greater than or equal to 0.2 eV. A difference in LUMO level between the organic compounds is preferably greater than or equal to 0.05 eV, further preferably greater than or equal to 0.1 eV, and still further preferably greater than or equal to 0.2 eV.

FIG. 1C shows a correlation of energy levels of the organic compound 131_1, the organic compound 131_2, and the guest material 132 in the light-emitting layer 130.

The following explains what terms and signs in FIG. 1C represent:

Host (131_1): the organic compound 131_1;

Host (131_2): the organic compound 131_2;

Guest (132): the guest material 132 (the fluorescent compound);

S_(H): the S1 level of the organic compound 131_1 (the host material);

T_(H): the T1 level of the organic compound 131_1 (the host material);

S_(G): the S1 level of the guest material 132 (the fluorescent compound);

T_(G): the T1 level of the guest material 132 (the fluorescent compound);

S_(E): the S1 level of the exciplex; and

T_(E): the T1 level of the exciplex.

In the light-emitting element of one embodiment of the present invention, the organic compounds 131_1 and 131_2 included in the light-emitting layer 130 form an exciplex. The lowest singlet excited energy level of the exciplex (S_(E)) and the lowest triplet excited energy level of the exciplex (T_(E)) are adjacent to each other (see Route E₃ in FIG. 1C).

An exciplex is an excited state formed from two kinds of substances. In photoexcitation, the exciplex is formed by interaction between one substance in an excited state and the other substance in a ground state. The two kinds of substances that have formed the exciplex return to a ground state by emitting light and then serve as the original two kinds of substances. In electrical excitation, when one substance is brought into an excited state, the one immediately interacts with the other substance to form an exciplex. Alternatively, one substance receives a hole and the other substance receives an electron to readily form an exciplex. In this case, any of the substances can form an exciplex without forming an excited state and; accordingly, most excitons in the light-emitting layer 130 can exist as exciplexes. Because the excited energy levels of the exciplex (S_(E) and T_(E)) are lower than the singlet excited energy level of the host materials (S_(H)) (the organic compound 131_1 and the organic compound 131_2) that form the exciplex, the excited state of the host material 131 can be formed with lower excited energy. Accordingly, the driving voltage of the light-emitting element 250 can be reduced.

Since the singlet excited energy level (S_(E)) and the triplet excited energy level (T_(E)) of the exciplex are adjacent to each other, the exciplex has a function of exhibiting thermally activated delayed fluorescence. In other words, the exciplex has a function of converting triplet excited energy to singlet excited energy by reverse intersystem crossing (upconversion) (see Route E₄ in FIG. 1C). Thus, the triplet excited energy generated in the light-emitting layer 130 is partly converted into singlet excited energy by the exciplex. In order to cause this conversion, the energy difference between the singlet excited energy level (S_(E)) and the triplet excited energy level (T_(E)) of the exciplex is preferably greater than 0 eV and less than or equal to 0.2 eV. Note that in order to efficiently make reverse intersystem crossing occur, the triplet excited energy level of the exciplex (T_(E)) is preferably lower than the triplet excited energy levels of the organic compounds (the organic compound 131_1 and the organic compound 131_2) in the host material which form the exciplex. Thus, quenching of the triplet excited energy of the exciplex due to the organic compounds is less likely to occur, which causes reverse intersystem crossing efficiently.

Furthermore, the singlet excited energy level of the exciplex (S_(E)) is preferably higher than the singlet excited energy level of the guest material 132 (S_(G)). In this way, the singlet excited energy of the formed exciplex can be transferred from the singlet excited energy level of the exciplex (S_(E)) to the singlet excited energy level of the guest material 132 (S_(G)), so that the guest material 132 is brought into the singlet excited state, causing light emission (see Route E₅ in FIG. 1C).

To obtain efficient light emission from the singlet excited state of the guest material 132, the fluorescence quantum yield of the guest material 132 is preferably high, and specifically, 50% or higher, further preferably 70% or higher, still further preferably 90% or higher.

Note that since direct transition from a singlet ground state to a triplet excited state in the guest material 132 is forbidden, energy transfer from the singlet excited energy level of the exciplex (S_(E)) to the triplet excited energy level of the guest material 132 (T_(G)) is unlikely to be a main energy transfer process.

When transfer of the triplet excited energy from the triplet excited energy level of the exciplex (T_(E)) to the triplet excited energy level of the guest material 132 (T_(G)) occurs, the triplet excited energy is deactivated (see Route E₆ in FIG. 1C). Thus, it is preferable that the energy transfer of Route E₆ be less likely to occur because the efficiency of generating the triplet excited state of the guest material 132 can be decreased and thermal deactivation can be reduced. In order to make this condition, the weight ratio of the guest material 132 to the host material 131 is preferably low, specifically, preferably greater than or equal to 0.001 and less than or equal to 0.05, further preferably greater than or equal to 0.001 and less than or equal to 0.01.

Note that when the direct carrier recombination process in the guest material 132 is dominant, a large number of triplet excitons are generated in the light-emitting layer 130, resulting in decreased emission efficiency due to thermal deactivation. Thus, it is preferable that the probability of the energy transfer process through the exciplex formation process (Routes E₄ and E₅ in FIG. 1C) be higher than the probability of the direct carrier recombination process in the guest material 132 because the efficiency of generating the triplet excited state of the guest material 132 can be decreased and thermal deactivation can be reduced. Therefore, as described above, the weight ratio of the guest material 132 to the host material 131 is preferably low, specifically, preferably greater than or equal to 0.001 and less than or equal to 0.05, further preferably greater than or equal to 0.001 and less than or equal to 0.01.

By making all the energy transfer processes of Routes E₄ and E₅ efficiently occur in the above-described manner, both the singlet excited energy and the triplet excited energy of the host material 131 can be efficiently converted into the singlet excited energy of the guest material 132, whereby the light-emitting element 250 can emit light with high emission efficiency.

The above-described processes through Routes E₃, E₄, and E₅ may be referred to as exciplex-singlet energy transfer (ExSET) or exciplex-enhanced fluorescence (ExEF) in this specification and the like. In other words, in the light-emitting layer 130, excited energy is given from the exciplex to the guest material 132.

When the light-emitting layer 130 has the above-described structure, light emission from the guest material 132 of the light-emitting layer 130 can be obtained efficiently.

<Energy Transfer Mechanism>

Next, factors controlling the processes of intermolecular energy transfer between the host material 131 and the guest material 132 will be described. As mechanisms of the intermolecular energy transfer, two mechanisms, i.e., Förster mechanism (dipole-dipole interaction) and Dexter mechanism (electron exchange interaction), have been proposed. Although the intermolecular energy transfer process between the host material 131 and the guest material 132 is described here, the same can apply to a case where the host material 131 is an exciplex.

<<Förster Mechanism>>

In Förster mechanism, energy transfer does not require direct contact between molecules and energy is transferred through a resonant phenomenon of dipolar oscillation between the host material 131 and the guest material 132. By the resonant phenomenon of dipolar oscillation, the host material 131 provides energy to the guest material 132, and thus, the host material 131 in an excited state is brought to a ground state and the guest material 132 in a ground state is brought to an excited state. Note that the rate constant k_(h*→g) of Förster mechanism is expressed by Formula (1).

$\begin{matrix} {k_{h^{*}\rightarrow g} = {\frac{9000\; c^{4}K^{2}\phi\;\ln\; 10}{128\;\pi^{5}n^{4}N\;\tau\; R^{6}}{\int{\frac{{f_{h}^{\prime}(v)}{ɛ_{g}(v)}}{v^{4}}d\; v}}}} & (1) \end{matrix}$

In Formula (1), v denotes a frequency, f′_(h)(v) denotes a normalized emission spectrum of the host material 131 (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), ϵ_(g)(v) denotes a molar absorption coefficient of the guest material 132, N denotes Avogadro's number, n denotes a refractive index of a medium, R denotes an intermolecular distance between the host material 131 and the guest material 132, τ denotes a measured lifetime of an excited state (fluorescence lifetime or phosphorescence lifetime), c denotes the speed of light, φ denotes a luminescence quantum yield (a fluorescence quantum yield in energy transfer from a singlet excited state, and a phosphorescence quantum yield in energy transfer from a triplet excited state), and K² denotes a coefficient (0 to 4) of orientation of a transition dipole moment between the host material 131 and the guest material 132. Note that K² is ⅔ in random orientation.

<<Dexter Mechanism>>

In Dexter mechanism, the host material 131 and the guest material 132 are close to a contact effective range where their orbitals overlap, and the host material 131 in an excited state and the guest material 132 in a ground state exchange their electrons, which leads to energy transfer. Note that the rate constant k_(h*→g) of Dexter mechanism is expressed by Formula (2).

$\begin{matrix} {k_{h^{*}\rightarrow g} = {\left( \frac{2\;\pi}{h} \right)K^{2}{\exp\left( {- \frac{2\; R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}d\; v}}}} & (2) \end{matrix}$

In Formula (2), h denotes a Planck constant, K denotes a constant having an energy dimension, v denotes a frequency, f′_(h)(v) denotes a normalized emission spectrum of the host material 131 (a fluorescent spectrum in energy transfer from a singlet excited state, and a phosphorescent spectrum in energy transfer from a triplet excited state), ϵ′_(g)(v) denotes a normalized absorption spectrum of the guest material 132, L denotes an effective molecular radius, and R denotes an intermolecular distance between the host material 131 and the guest material 132.

Here, the efficiency of energy transfer from the host material 131 to the guest material 132 (energy transfer efficiency φ_(ET)) is expressed by Formula (3). In the formula, k_(r) denotes a rate constant of a light-emission process (fluorescence in energy transfer from a singlet excited state, and phosphorescence in energy transfer from a triplet excited state) of the host material 131, k_(n) denotes a rate constant of a non-light-emission process (thermal deactivation or intersystem crossing) of the host material 131, and τ denotes a measured lifetime of an excited state of the host material 131.

$\begin{matrix} {\phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & (3) \end{matrix}$

According to Formula (3), it is found that the energy transfer efficiency φ_(ET) can be increased by increasing the rate constant k_(h*→g) of energy transfer so that another competing rate constant k_(r)+k_(n) (=1/τ) becomes relatively small.

<<Concept for Promoting Energy Transfer>>

First, an energy transfer by Förster mechanism is considered. When Formula (1) is substituted into Formula (3), τ can be eliminated. Thus, in Förster mechanism, the energy transfer efficiency φ_(ET) does not depend on the lifetime τ of the excited state of the host material 131. In addition, it can be said that the energy transfer efficiency φ_(ET) is higher when the luminescence quantum yield φ (here, the fluorescence quantum yield because energy transfer from a singlet excited state is discussed) is higher. In general, the luminescence quantum yield of an organic compound in a triplet excited state is extremely low at room temperature. Thus, in the case where the host material 131 is in a triplet excited state, a process of energy transfer by Förster mechanism can be ignored, and a process of energy transfer by Förster mechanism is considered only in the case where the host material 131 is in a singlet excited state.

Furthermore, it is preferable that the emission spectrum (the fluorescent spectrum in the case where energy transfer from a singlet excited state is discussed) of the host material 131 largely overlap with the absorption spectrum (absorption corresponding to the transition from the singlet ground state to the singlet excited state) of the guest material 132. Moreover, it is preferable that the molar absorption coefficient of the guest material 132 be also high. This means that the emission spectrum of the host material 131 overlaps with the absorption band of the guest material 132 which is on the longest wavelength side. Since direct transition from the singlet ground state to the triplet excited state of the guest material 132 is forbidden, the molar absorption coefficient of the guest material 132 in the triplet excited state can be ignored. Thus, a process of energy transfer to a triplet excited state of the guest material 132 by Förster mechanism can be ignored, and only a process of energy transfer to a singlet excited state of the guest material 132 is considered. That is, in Förster mechanism, a process of energy transfer from the singlet excited state of the host material 131 to the singlet excited state of the guest material 132 is considered.

Next, an energy transfer by Dexter mechanism is considered. According to Formula (2), in order to increase the rate constant k_(h*→g), it is preferable that an emission spectrum of the host material 131 (a fluorescent spectrum in the case where energy transfer from a singlet excited state is discussed) largely overlap with an absorption spectrum of the guest material 132 (absorption corresponding to transition from a singlet ground state to a singlet excited state). Therefore, the energy transfer efficiency can be optimized by making the emission spectrum of the host material 131 overlap with the absorption band of the guest material 132 which is on the longest wavelength side.

When Formula (2) is substituted into Formula (3), it is found that the energy transfer efficiency φ_(ET) in Dexter mechanism depends on τ. In Dexter mechanism, which is a process of energy transfer based on the electron exchange, as well as the energy transfer from the singlet excited state of the host material 131 to the singlet excited state of the guest material 132, energy transfer from the triplet excited state of the host material 131 to the triplet excited state of the guest material 132 occurs.

In the light-emitting element of one embodiment of the present invention in which the guest material 132 is a fluorescent material, the efficiency of energy transfer to the triplet excited state of the guest material 132 is preferably low. That is, the energy transfer efficiency based on Dexter mechanism from the host material 131 to the guest material 132 is preferably low and the energy transfer efficiency based on Förster mechanism from the host material 131 to the guest material 132 is preferably high.

As described above, the energy transfer efficiency in Förster mechanism does not depend on the lifetime τ of the excited state of the host material 131. In contrast, the energy transfer efficiency in Dexter mechanism depends on the excitation lifetime τ of the host material 131. To reduce the energy transfer efficiency in Dexter mechanism, the excitation lifetime τ of the host material 131 is preferably short.

The rate constant of Förster mechanism is inversely proportional to the 6th power of the distance between the host material 131 and the guest material 132, and the rate constant of Dexter mechanism is inversely proportional to the exponential function of the distance between the host material 131 and the guest material 132. Thus, when the distance between the two molecules is approximately 1 nm or less, Dexter mechanism is dominant, and when the distance is approximately 1 nm or more, Förster mechanism is dominant. To reduce the energy transfer efficiency in Dexter mechanism, the distance between the host material 131 and the guest material 132 is preferably large, and specifically, 0.7 nm or more, further preferably 0.9 nm or more, still further preferably 1 nm or more. Furthermore, to efficiently cause the energy transfer by Förster mechanism, the distance between the host material 131 and the guest material 132 is preferably 5 nm or less.

<Distance Between Host Material and Guest Material>

To increase the distance between the host material 131 and the guest material 132, or more specifically, to increase the distance between a region relating to energy transfer of the host material 131 and a region relating to energy transfer of the guest material 132, at least one of the host material 131 and the guest material 132 preferably has a substituent that inhibits the proximity between the host material 131 and the guest material 132.

In the light-emitting layer 130, the host material 131 is present in the largest proportion, and the guest material 132 is dispersed in the host material 131. Thus, the guest material 132 preferably has a substituent that prevents the proximity to the host material 131.

Note that it is preferable that the substituent hardly relate to the energy transfer between the host material 131 and the guest material 132. That is, in the substituent, the distribution of molecular orbitals and the spin density are preferably low in the singlet excited state and the triplet excited state. The substituent is preferably aliphatic hydrocarbon, further preferably an alkyl group, still further preferably a branched alkyl group.

Thus, in the light-emitting element of one embodiment of the present invention, the guest material 132 preferably includes at least two alkyl groups each having 2 or more carbon atoms. Alternatively, the guest material 132 preferably includes at least two branched alkyl groups each having 3 to 10 carbon atoms. Alternatively, the guest material 132 preferably includes at least two cyclic hydrocarbon groups each having 3 to 10 carbon atoms or at least two bridged cyclic hydrocarbon groups each having 3 to 10 carbon atoms. Furthermore, the guest material 132 preferably includes condensed aromatic hydrocarbon having 3 to 12 carbon atoms.

The alkyl group having 2 or more carbon atoms may be either a strain-chain alkyl group or a branched alkyl group, and an alkyl group having 2 to 10 carbon atoms, such as an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, or a decyl group is preferably used. As the alkyl group, in particular, a branched alkyl group having 3 to 10 carbon atoms, such as an isopropyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an isopentyl group, a neopentyl group, a tert-pentyl group, an isohexyl group, a 2-ethylhexyl group, or a 2-ethyloctyl group is preferably used. As the cyclic hydrocarbon group having 3 to 10 carbon atoms or the bridged cyclic hydrocarbon group having 3 to 10 carbon atoms, a cyclic hydrocarbon group such as a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, or a cyclohexyl group or a bridged cyclic hydrocarbon group such as a norbornyl group, a bornyl group, a fenchyl group, a bicyclo[2.2.2]octanyl group, a bicyclo[3.2.1]octanyl group, or an adamantyl group can be given. As the condensed aromatic hydrocarbon having 3 to 12 carbon atoms, which is preferably used for the guest material, an anthracene skeleton, a phenanthren skeleton, a fluorine skeleton, an indacene skeleton, a tetracene skeleton, a triphenylen skeleton, a chrysene skeleton, a pyrene skeleton, a fluoranthene skeleton, a pentacene skeleton, a perylene skeleton, a rubicene skeleton, a trinaphthylene skeleton, a dibenzoperiflanthene (or dibenzo[5,6]indeno[1,2,3-cd:1′,2′,3′-lm]perylene) skeleton, or the like is preferably used, and in particular, condensed aromatic hydrocarbon having 3 to 6 carbon atoms is preferably used.

Next, an example of calculating the distance between the host material and the guest material which can be favorably used in the light-emitting element of one embodiment of the present invention is shown. This calculation was conducted using a classical molecular dynamics method. The structures and abbreviations of compounds used in the calculation are shown below. In addition, the calculation conditions are shown in Table 1.

TABLE 1 Software SCIGRESS MD-ME Calculation method classical molecular dynamics: CMD Potential Lennard-Jones VdW potential (DREIDING and OPLS) Atomic charge Approximation of the Electrostatic Potential Cutoff distance 1 nm Ensemble NTP (constant temperature and pressure) Time interval 0.2 fs Relaxation time 1 ns Number of 100 molecules in cell (host: 99, guest: 1) Number of 7048 in TBRb atoms in cell 7000 in Rubrene

The calculation was performed by the following method. Note that simulation software “SCIGRESS ME2.0” manufactured by Fujitsu Limited was used for classical molecular dynamics calculation. As potential, DREIDING and OPLS, which is Lennard-Jones VdW potential, was used. Note that the calculation was performed using a high performance computer (C2112, manufactured by SGI Japan, Ltd.).

As a calculation model, a standard cell containing 99 molecules of 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm) as a host material and one molecule of 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (abbreviation: TBRb) or one molecule of Rubrene as a guest material was used. As the initial structure of the molecule structure of each material, the most stable structure (singlet ground state) obtained from the first-principles calculation was used. The electric charge distribution of each molecule was obtained by performing electrostatic potential approximation on the basis of the results obtained from the first-principles calculation, and it was set as “atomic charges”.

For the first-principles calculation, Gaussian 09, which is the quantum chemistry computational program, was used, and the most stable structure in the singlet ground state was calculated by the density functional theory (DFT). As a basis function, 6-311G(d,p) was used, and as a functional, B3LYP was used. Note that a high performance computer (ICE X, manufactured by SGI Japan, Ltd.) was used for the calculation.

The above initial structure was subjected to calculation for a relaxation time of 1 ns which is sufficient relative to a time interval of 0.2 fs that reproduces molecular vibration; thus, an amorphous structure was obtained. FIG. 2 shows a part of a standard cell with the amorphous structure containing the host material (4,6mCzP2Pm) and the guest material (Rubrene), which was obtained by the above method. In FIG. 2, for simplicity, Rubrene, which is the guest material, is shown in space filling display, and 4,6mCzP2Pm, which is the host material, is shown in stick display.

As shown in FIG. 2, a large number of host material molecules exist in the vicinity of the guest material molecule.

Then, to analyze the obtained amorphous structure, coordinate data for a certain period (150 ps) were added up. From the coordinate data, the running coordination number of the host material around the guest material was calculated on the basis of the centroids of the molecules.

Note that the running coordination number refers to the number of atomic species j which exist in a sphere with a center at an atomic species i and a radius of r. The running coordination number N_(ij)(r) can be represented using pair correlation function g_(ij)(r).

The pair correlation function g_(ij)(r) refers to the average number of atomic species j per unit volume which exist in a sphere with a center at an atomic species i and a radius of r. Note that the pair correlation function g_(ij)(r) is represented as the average number of atoms per average density N_(j)/V. When the total number of atomic species i contained in the system is N_(j), the volume is V, and the number of atomic species k contained in a spherical shell with a thickness Δr at a distance r is n_(ik), the pair correlation function g_(ij)(r) is represented by Formula (4). In addition, the running coordination number N_(ij)(r) is represented by Formula (5). In the case where the atomic species i and the atomic species j are the same kind of atoms, N_(j) is equal to N_(i)−1.

$\begin{matrix} {{g_{ij}(r)} = {\frac{V}{N_{i}N_{j}}{\sum\limits_{k}^{N_{i}}\frac{n_{ik}\left( {{r - \frac{\Delta\; r}{2}},{r + \frac{\Delta\; r}{2}}} \right)}{4\;\pi\; r^{2}\Delta\; r}}}} & (4) \\ {{N_{ij}(r)} = {\sum\limits_{n}{\frac{N_{j}}{V}{g_{ij}\left( {n\;\Delta\; r} \right)}}}} & (5) \end{matrix}$

In this embodiment, the pair correlation function g_(ij)(r) was calculated by averaging the plurality of time series data and averaging the number of atoms. In the calculation of the running coordination number N_(ij)(r), n is the number until n·Δr reached r. The running coordination number N_(ij)(r) was also calculated by averaging the plurality of time series data and averaging the number of atoms.

FIG. 3 shows the running coordination number calculated by the above method. In FIG. 3, the vertical axis represents the running coordination number of the host material molecules around the guest material molecule, and the horizontal axis represents the distance between the centroids of the host material molecules and the guest material molecule. From the distance between the centroids when the running coordination number exceeds 0, the distance between the guest material molecule and the host material molecule positioned at the nearest distance from the guest material molecule can be obtained. In the case where the guest material is Rubrene, the distance between the centroids of the Rubrene molecule and the host material (4,6mCzP2Pm) molecule positioned at the nearest distance from the Rubrene molecule is 0.59 nm. In the case where the guest material is TBRb, the distance between the centroids of the TBRb molecule and the host material (4,6mCzP2Pm) molecule positioned at the nearest distance from the TBRb molecule is 0.90 nm, which is larger than that in the case where the guest material is Rubrene. Furthermore, the number of the host material molecules existing within 1 nm from the guest material molecule is six when Rubrene is used, and is two when TBRb is used. Therefore, when TBRb is used for the guest material, the distance between the host material and the guest material becomes long, so that the energy transfer efficiency between the host material and the guest material based on Dexter mechanism can be reduced. From this result, the number of the host material molecules existing within 1 nm from the guest material molecule is preferably 5 or less, further preferably 4 or less, still further preferably 2 or less.

Two host material molecules close to TBRb within 1 nm are close to a position where there is no tert-butyl group (which is placed in a direction substantially perpendicular to a surface including a tetracene skeleton and a phenyl group) in the molecule structure of TBRb. Thus, at the position where TBRb includes a tert-butyl group, the host material is apart from TBRb by more than 1 nm; that is, the proximity between the guest material and the host material is prevented by the tert-butyl group.

Next, FIGS. 4A and 4B each illustrate, in the calculated standard cell, the guest material molecule (TBRb or Rubrene) and the host material molecule (4,6mCzP2Pm) positioned at the nearest distance from the guest material molecule. FIG. 4A illustrates two proximate molecules in the case where the guest material molecule is TBRb and the host material molecule is 4,6mCzP2Pm, and FIG. 4B illustrates two proximate molecules in the case where the guest material molecule is Rubrene and the host material molecule is 4,6mCzP2Pm.

As illustrated in FIG. 4B, in the case where the guest material molecule is Rubrene, a hydrogen atom is positioned at a terminal of a tetracene skeleton, so that the distance between the guest material molecule and the host material molecule (4,6mCzP2Pm) is short. In contrast, as illustrated in FIG. 4A, in the case where the guest material molecule is TBRb, tert-butyl groups are positioned at a terminal of a tetracene skeleton, so that the distance between the host material molecule (4,6mCzP2Pm) and the tetracene skeleton in TBRb is long.

Furthermore, FIGS. 5A to 5C show calculation results of the most stable structures and the spin density distribution of the host material (4,6mCzP2Pm) and the guest material (TBRb and Rubrene) in the triplet excited state, obtained by using first-principles calculation. For the first-principles calculation, Gaussian 09, which is the quantum chemistry computational program, was used, and the most stable structure in the triplet excited state was calculated by the density functional theory (DFT). As a basis function, 6-311G(d,p) was used, and as a functional, B3LYP was used. FIGS. 5A, 5B, and 5C illustrate the most stable structure and the spin density distribution of 4,6mCzP2Pm, TBRb, and Rubrene in the triplet excited state, respectively.

As illustrated in FIGS. 5B and 5C, the spin density distribution of TBRb and Rubrene is localized in the tetracene skeleton, and the spin density distribution of a tert-butyl group included in TBRb is small. Thus, even when the tert-butyl group in TBRb is close to 4,6mCzP2Pm, which is the host material molecule, the energy transfer between the host material and the guest material is hardly affected. That is, it is important that the distance between a skeleton which has high spin density distribution (here, a tetracene skeleton) in the guest material and the host material is long.

Note that the guest material preferably includes at least two substituents that prevent the proximity to the host material because the proximity between the guest material and the host material can be effectively prevented.

Thus, in one embodiment of the present invention, the guest material 132 preferably includes at least two alkyl groups each having 2 or more carbon atoms. Alternatively, the guest material 132 preferably includes at least two branched alkyl groups each having 3 to 10 carbon atoms. Alternatively, the guest material 132 preferably includes at least two cyclic hydrocarbon groups each having 3 to 10 carbon atoms or at least two bridged cyclic hydrocarbon groups each having 3 to 10 carbon atoms. Furthermore, the guest material 132 preferably includes condensed aromatic hydrocarbon having 3 to 12 carbon atoms.

In a manner similar to that of the energy transfer from the host material 131 to the guest material 132, the energy transfer by both Förster mechanism and Dexter mechanism also occurs in the energy transfer process from the exciplex to the guest material 132.

Accordingly, one embodiment of the present invention provides a light-emitting element including, as the host material 131, the organic compound 131_1 and the organic compound 131_2 which are a combination for forming an exciplex which functions as an energy donor capable of efficiently transferring energy to the guest material 132. The exciplex formed by the organic compound 131_1 and the organic compound 131_2 has a singlet excited energy level and a triplet excited energy level which are adjacent to each other; accordingly, transition from a triplet exciton generated in the light-emitting layer 130 to a singlet exciton (reverse intersystem crossing) is likely to occur. This can increase the efficiency of generating singlet excitons in the light-emitting layer 130. Furthermore, in order to facilitate energy transfer from the singlet excited state of the exciplex to the singlet excited state of the guest material 132 having a function as an energy acceptor, it is preferable that the emission spectrum of the exciplex overlap with the absorption band of the guest material 132 which is on the longest wavelength side (lowest energy side). Thus, the efficiency of generating the singlet excited state of the guest material 132 can be increased. In addition, the guest material 132 includes at least two substituents that prevent the proximity to the exciplex, whereby the energy transfer efficiency from the triplet excited state of the exciplex to the triplet excited state of the guest material 132 can be reduced and the generation efficiency of the singlet excited state can be improved.

In addition, fluorescence lifetime of a thermally activated delayed fluorescence component in light emitted from the exciplex is preferably short, and specifically, preferably 10 ns or longer and 50 μs or shorter, further preferably 10 ns or longer and 20 μs or shorter, still further preferably 10 ns or longer and 10 μs or shorter.

The proportion of a thermally activated delayed fluorescence component in the light emitted from the exciplex is preferably high. Specifically, the proportion of a thermally activated delayed fluorescence component in the light emitted from the exciplex is preferably higher than or equal to 10%, further preferably higher than or equal to 30%, still further preferably higher than or equal to 50%.

<Material>

Next, components of a light-emitting element of one embodiment of the present invention are described in detail below.

<<Light-emitting Layer>>

Next, materials that can be used for the light-emitting layer 130 will be described below.

In the light-emitting layer 130, the host material 131 is present in the largest proportion by weight, and the guest material 132 (the fluorescent material) is dispersed in the host material 131. The S1 level of the host material 131 (the organic compound 131_1 and the organic compound 131_2) in the light-emitting layer 130 is preferably higher than the S1 level of the guest material 132 (the fluorescent material) in the light-emitting layer 130. The T1 level of the host material 131 (the organic compound 131_1 and the organic compound 131_2) in the light-emitting layer 130 is preferably higher than the T1 level of the guest material 132 (the fluorescent material) in the light-emitting layer 130.

In the light-emitting layer 130, the guest material 132 preferably includes condensed aromatic hydrocarbon having 3 to 12 carbon atoms. Furthermore, the guest material 132 preferably includes at least two alkyl groups each having 2 or more carbon atoms. Alternatively, the guest material 132 preferably includes at least two branched alkyl groups each having 3 to 10 carbon atoms. Alternatively, the guest material 132 preferably includes at least two cyclic hydrocarbon groups each having 3 to 10 carbon atoms or at least two bridged cyclic hydrocarbon groups each having 3 to 10 carbon atoms.

When the guest material 132 has the above structure, the minimum value of the distance between centroids of the host material 131 and the guest material 132 can be 0.7 nm or more and 5 nm or less, and the energy transfer in Dexter mechanism between the host material 131 and the guest material 132 can be inhibited. Thus, the energy transfer efficiency from the triplet excited state of the host material 131 to the triplet excited state of the guest material 132 can be reduced.

As examples of the guest material having the above structure, a tetracene derivative, a perylene derivative, a pyrene derivative, a coumarin derivative, a phenoxazine derivative, a phenothiazine derivative, an anthracene derivative, a chrysene derivative, a phenanthrene derivative, a stilbene derivative, an acridone derivative, and the like, such as 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (abbreviation: TBRb), 2,5,8,11-tetra(tert-butyl)perylene (abbreviation: TBP), coumarin 6, coumarin 30, Nile red, N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-bis(4-tert-butylphenyl)pyrene-1,6-diamine (abbreviation: 1,6tBu-FLPAPrn), and N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenyl-3,8-dicyclohexylpyrene-1, 6-diamine (abbreviation: ch-1,6FLPAPrn) are given.

Examples of the organic compound 131_1 include a zinc- or aluminum-based metal complex, an oxadiazole derivative, a triazole derivative, a benzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a pyrimidine derivative, a triazine derivative, a pyridine derivative, a bipyridine derivative, a phenanthroline derivative, and the like. Other examples are an aromatic amine, a carbazole derivative, and the like.

Any of the following hole-transport materials and electron-transport materials can be used.

A material having a property of transporting more holes than electrons can be used as the hole-transport material, and a material having a hole mobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, an aromatic amine, a carbazole derivative, an aromatic hydrocarbon, a stilbene derivative, or the like can be used. Furthermore, the hole-transport material may be a high molecular compound.

Examples of the material having a high hole-transport property are N,N′-di(p-tolyl)-N,N-diphenyl-p-phenylenediamine (abbreviation: DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation: DPAB), N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-diphenyl-(1,1′-biphenyl)-4,4′-diamine (abbreviation: DNTPD), 1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene (abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivative are 3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA1), 3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzDPA2), 3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole (abbreviation: PCzTPN2), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), 3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA2), 3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole (abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivative are 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA), 1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and the like.

Examples of the aromatic hydrocarbon are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene, 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene (abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA), 2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene, 9,10-bis[2-(1-naphthyl)phenyl]anthracene, 2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene, 2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl, 10,10′-diphenyl-9,9′-bianthryl, 10,10′-bis(2-phenylphenyl)-9,9′-bianthryl, 10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene, tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, and the like. Other examples are pentacene, coronene, and the like. The aromatic hydrocarbon having a hole mobility of 1×10⁻⁶ cm²/Vs or higher and having 14 to 42 carbon atoms is particularly preferable.

The aromatic hydrocarbon may have a vinyl skeleton. Examples of the aromatic hydrocarbon having a vinyl group are 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA), and the like.

Other examples are high molecular compounds such as poly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA), poly[N-(4-{N′[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide] (abbreviation: PTPDMA), and poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation: poly-TPD).

Examples of the material having a high hole-transport property are aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine (abbreviation: TCTA), 4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation: 1′-TNATA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation: TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (abbreviation: MTDATA), 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: mBPAFLP), N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine (abbreviation: DFLADFL), N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine (abbreviation: DPNF), 2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: DPASF), 4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBBi1BP), 4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBANB), 4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBNBB), 4-phenyldiphenyl-(9-phenyl-9H-carbazol-3-yl)amine (abbreviation: PCA1BP), N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine (abbreviation: PCA2B), N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine (abbreviation: PCA3B), N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine (abbreviation: PCBiF), N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF), 9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine (abbreviation: PCBAF), N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine (abbreviation: PCBASF), 2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene (abbreviation: PCASF), 2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene (abbreviation: DPA2SF), N-[4-(9H-carbazol-9-yl)phenyl]-N-(4-phenyl)phenylaniline (abbreviation: YGA1BP), and N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine (abbreviation: YGA2F). Other examples are amine compounds, carbazole compounds, thiophene compounds, furan compounds, fluorene compounds; triphenylene compounds; phenanthrene compounds, and the like such as 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN), 3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn), 3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP), 1,3-bis(N-carbazolyl)benzene (abbreviation: mCP), 3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran (abbreviation: mmDBFFLBi-II), 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II), 1,3,5-tri(dibenzothiophen-4-yl)-benzene (abbreviated as DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene (abbreviation: DBTFLP-III), 4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene (abbreviation: DBTFLP-IV), and 4-[3-(triphenylene-2-yl)phenyl]dibenzothiophene (abbreviation: mDBTPTp-II). The substances described here are mainly substances having a hole mobility of 1×10⁻⁶ cm²/Vs or higher. Note that other than these substances, any substance that has a property of transporting more holes than electrons may be used.

As the electron-transport material, a material having a property of transporting more electrons than holes can be used, and a material having an electron mobility of 1×10⁻⁶ cm²/Vs or higher is preferable. A π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound, a metal complex, or the like can be used as the material which easily accepts electrons (the material having an electron-transport property). Specific examples include a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, and a thiazole ligand. Other examples include an oxadiazole derivative, a triazole derivative, a phenanthroline derivative, a pyridine derivative, a bipyridine derivative, a pyrimidine derivative, and the like.

Examples include metal complexes having a quinoline or benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(III) (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq) and bis(8-quinolinolato)zinc(II) (abbreviation: Znq), and the like. Alternatively, a metal complex having an oxazole-based or thiazole-based ligand, such as bis[2-(2-benzoxazolyl)phenolate]zinc(II) (abbreviation: ZnPBO) or bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ) can be used. Other than such metal complexes, any of the following can be used: heterocyclic compounds such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation: CO11), 3-(biphenyl-4-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI), 2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole (abbreviation: mDBTBIm-II), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), and 2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation: NBPhen); heterocyclic compounds having a diazine skeleton such as 2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTPDBq-II), 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mDBTBPDBq-II), 2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline (abbreviation: 2mCzBPDBq), 2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 2CzPDBq-III), 7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 7mDBTPDBq-II), 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation: 6mDBTPDBq-II), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation: 4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation: 4,6mDBTP2Pm-II), and 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm); heterocyclic compounds having a triazine skeleton such as 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine (abbreviation: PCCzPTzn); heterocyclic compounds having a pyridine skeleton such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy); and heteroaromatic compounds such as 4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Further alternatively, a high molecular compound such as poly(2,5-pyridinediyl) (abbreviation: PPy), poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), or poly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used. The substances described here are mainly substances having an electron mobility of 1×10⁻⁶ cm²/Vs or higher. Note that other substances may also be used as long as their electron-transport properties are higher than their hole-transport properties.

As the organic compound 131_2, a substance which can form an exciplex together with the organic compound 131_1 is preferably used. Specifically, any of the hole-transport materials and the electron-transport materials described above can be used. In that case, it is preferable that the organic compound 131_1, the organic compound 131_2, and the guest material 132 (the fluorescent material) be selected such that the emission peak of the exciplex formed by the organic compound 131_1 and the organic compound 131_2 overlaps with an absorption band on the longest wavelength side (lowest energy side) of the guest material 132 (the fluorescent material). This makes it possible to provide a light-emitting element with drastically improved emission efficiency.

As the host material 131 (the organic compound 131_1 and the organic compound 131_2) included in the light-emitting layer 130, a material having a function of converting triplet excited energy into singlet excited energy is preferable. As the material having a function of converting triplet excited energy into singlet excited energy, a thermally activated delayed fluorescent (TADF) material can be given in addition to the exciplex. Therefore, the term “exciplex” in the description can be replaced with the term “thermally activated delayed fluorescence material”. Note that the thermally activated delayed fluorescence material is a material having a small difference between the triplet excited energy level and the singlet excited energy level and a function of converting triplet excited energy into singlet excited energy by reverse intersystem crossing. Thus, the thermally activated delayed fluorescence material can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing) using a little thermal energy and efficiently exhibit light emission (fluorescence) from the singlet excited state. Thermally activated delayed fluorescence is efficiently obtained under the condition where the difference between the triplet excited energy level and the singlet excited energy level is more than 0 eV and less than or equal to 0.2 eV, preferably more than 0 eV and less than or equal to 0.1 eV.

The material that exhibits thermally activated delayed fluorescence may be a material that can form a singlet excited state by itself from a triplet excited state by reverse intersystem crossing. In the case where the thermally activated delayed fluorescence material is composed of one kind of material, any of the following materials can be used, for example.

First, a fullerene, a derivative thereof, an acridine derivative such as proflavine, eosin, and the like can be given. Furthermore, a metal-containing porphyrin, such as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd), can be given. Examples of the metal-containing porphyrin include a protoporphyrin-tin fluoride complex (SnF₂(Proto IX)), a mesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), a hematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrin tetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex (SnF₂(OEP)), an etioporphyrin-tin fluoride complex (SnF₂(Etio I)), and an octaethylporphyrin-platinum chloride complex (PtCl₂(OEP)).

As the thermally activated delayed fluorescence material composed of one kind of material, a heterocyclic compound having a π-electron rich heteroaromatic ring and a π-electron deficient heteroaromatic ring can be used. Specifically, 2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine (abbreviation: PIC-TRZ), 2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine (abbreviation: PCCzPTzn), 2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine (abbreviation: PXZ-TRZ), 3-[4-(5-phenyl-5,10-dihydrophenazine-10-yl)phenyl]-4,5-diphenyl-1,2,4-triazole (abbreviation: PPZ-3TPT), 3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation: ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (abbreviation: DMAC-DPS), or 10-phenyl-10H,10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation: ACRSA), can be used. The heterocyclic compound is preferable because of having the π-electron rich heteroaromatic ring and the π-electron deficient heteroaromatic ring, for which the electron-transport property and the hole-transport property are high. Note that a substance in which the π-electron rich heteroaromatic ring is directly bonded to the π-electron deficient heteroaromatic ring is particularly preferable because the donor property of the π-electron rich heteroaromatic ring and the acceptor property of the π-electron deficient heteroaromatic ring are both increased and the difference between the singlet excited level and the triplet excited level becomes small.

The light-emitting layer 130 can have a structure in which two or more layers are stacked. For example, in the case where the light-emitting layer 130 is formed by stacking a first light-emitting layer and a second light-emitting layer in this order from the hole-transport layer side, the first light-emitting layer is formed using a substance having a hole-transport property as the host material and the second light-emitting layer is formed using a substance having an electron-transport property as the host material.

The light-emitting layer 130 may include a material other than the host material 131 and the guest material 132.

<<Pair of Electrodes>>

The electrode 101 and the electrode 102 have functions of injecting holes and electrons into the light-emitting layer 130. The electrode 101 and the electrode 102 can be formed using a metal, an alloy, or a conductive compound, a mixture or a stack thereof, or the like. A typical example of the metal is aluminum (Al); besides, a transition metal such as silver (Ag), tungsten, chromium, molybdenum, copper, or titanium, an alkali metal such as lithium (Li) or cesium, or a Group 2 metal such as calcium or magnesium (Mg) can be used. As the transition metal, a rare earth metal such as ytterbium (Yb) may be used. An alloy containing any of the above metals can be used as the alloy, and MgAg and AlLi can be given as examples. Examples of the conductive compound include metal oxides such as indium tin oxide (hereinafter, referred to as ITO), indium tin oxide containing silicon or silicon oxide (ITSO), indium zinc oxide, indium oxide containing tungsten and zinc, and the like. It is also possible to use an inorganic carbon-based material such as graphene as the conductive compound. As described above, the electrode 101 and/or the electrode 102 may be formed by stacking two or more of these materials.

Light emitted from the light-emitting layer 130 is extracted through the electrode 101 and/or the electrode 102. Therefore, at least one of the electrodes 101 and 102 transmits visible light. As the conductive material transmitting light, a conductive material having a visible light transmittance higher than or equal to 40% and lower than or equal to 100%, preferably higher than or equal to 60% and lower than or equal to 100%, and a resistivity lower than or equal to 1×10⁻² Ω·cm can be used. The electrode on the light extraction side may be formed using a conductive material having functions of transmitting light and reflecting light. As the conductive material, a conductive material having a visible light reflectivity higher than or equal to 20% and lower than or equal to 80%, preferably higher than or equal to 40% and lower than or equal to 70%, and a resistivity lower than or equal to 1×10⁻² Ω·cm can be used. In the case where the electrode through which light is extracted is formed using a material with low light transmittance, such as metal or alloy, the electrode 101 and/or the electrode 102 is formed to a thickness that is thin enough to transmit visible light (e.g., a thickness of 1 nm to 10 nm).

In this specification and the like, as the electrode transmitting light, a material that transmits visible light and has conductivity is used. Examples of the material include, in addition to the above-described oxide conductor layer typified by an ITO, an oxide semiconductor layer and an organic conductor layer containing an organic substance. Examples of the organic conductive layer containing an organic substance include a layer containing a composite material in which an organic compound and an electron donor (donor material) are mixed and a layer containing a composite material in which an organic compound and an electron acceptor (acceptor material) are mixed. The resistivity of the transparent conductive layer is preferably lower than or equal to 1×10⁵ Ω·cm, further preferably lower than or equal to 1×10⁴ Ω·cm.

As the method for forming the electrode 101 and the electrode 102, a sputtering method, an evaporation method, a printing method, a coating method, a molecular beam epitaxy (MBE) method, a CVD method, a pulsed laser deposition method, an atomic layer deposition (ALD) method, or the like can be used as appropriate.

<<Hole-injection Layer>>

The hole-injection layer 111 has a function of reducing a barrier for hole injection from one of the pair of electrodes (the electrode 101 or the electrode 102) to promote hole injection and is formed using a transition metal oxide, a phthalocyanine derivative, or an aromatic amine, for example. As the transition metal oxide, molybdenum oxide, vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or the like can be given. As the phthalocyanine derivative, phthalocyanine, metal phthalocyanine, or the like can be given. As the aromatic amine, a benzidine derivative, a phenylenediamine derivative, or the like can be given. It is also possible to use a high molecular compound such as polythiophene or polyaniline; a typical example thereof is poly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which is self-doped polythiophene.

As the hole-injection layer 111, a layer containing a composite material of a hole-transport material and a material having a property of accepting electrons from the hole-transport material can also be used. Alternatively, a stack of a layer containing a material having an electron accepting property and a layer containing a hole-transport material may also be used. In a steady state or in the presence of an electric field, electric charge can be transferred between these materials. As examples of the material having an electron-accepting property, organic acceptors such as a quinodimethane derivative, a chloranil derivative, and a hexaazatriphenylene derivative can be given. A specific example is a compound having an electron-withdrawing group (a halogen group or a cyano group), such as 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation: F₄-TCNQ), chloranil, or 2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation: HAT-CN). Alternatively, a transition metal oxide such as an oxide of a metal from Group 4 to Group 8 can also be used. Specifically, vanadium oxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, rhenium oxide, or the like can be used. In particular, molybdenum oxide is preferable because it is stable in the air, has a low hygroscopic property, and is easily handled.

A material having a property of transporting more holes than electrons can be used as the hole-transport material, and a material having a hole mobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, any of the aromatic amine, carbazole derivative, aromatic hydrocarbon, stilbene derivative, and the like described as examples of the hole-transport material that can be used in the light-emitting layer 130 can be used. Furthermore, the hole-transport material may be a high molecular compound.

<<Hole-transport Layer>>

The hole-transport layer 112 is a layer containing a hole-transport material and can be formed using any of the materials given as examples of the material of the hole-injection layer 111. In order that the hole-transport layer 112 has a function of transporting holes injected into the hole-injection layer 111 to the light-emitting layer 130, the HOMO level of the hole-transport layer 112 is preferably equal or close to the HOMO level of the hole-injection layer 111.

As the hole-transport material, any of the materials given as examples of the material of the hole-injection layer 111 can be used. As the hole-transport material, a substance having a hole mobility of 1×10⁻⁶ cm²/Vs or higher is preferably used. Note that any substance other than the above substances may be used as long as the hole-transport property is higher than the electron-transport property. The layer including a substance having a high hole-transport property is not limited to a single layer, and two or more layers containing the aforementioned substances may be stacked.

<<Electron-transport Layer>>

The electron-transport layer 118 has a function of transporting, to the light-emitting layer 130, electrons injected from the other of the pair of electrodes (the electrode 101 or the electrode 102) through the electron-injection layer 119. A material having a property of transporting more electrons than holes can be used as the electron-transport material, and a material having an electron mobility of 1×10⁻⁶ cm²/Vs or higher is preferable. As the compound which easily accepts electrons (the material having an electron-transport property), a π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound, a metal complex, or the like can be used, for example. Specifically, a metal complex having a quinoline ligand, a benzoquinoline ligand, an oxazole ligand, or a thiazole ligand, which are described as the electron-transport materials that can be used in the light-emitting layer 130, can be given. Further, an oxadiazole derivative; a triazole derivative, a phenanthroline derivative, a pyridine derivative, a bipyridine derivative, a pyrimidine derivative, and the like can be given. A substance having an electron mobility of higher than or equal to 1×10⁻⁶ cm²/Vs is preferable. Note that other than these substances, any substance that has a property of transporting more electrons than holes may be used for the electron-transport layer. The electron-transport layer 118 is not limited to a single layer, and may include stacked two or more layers containing the aforementioned substances.

Between the electron-transport layer 118 and the light-emitting layer 130, a layer that controls transfer of electron carriers may be provided. This is a layer formed by addition of a small amount of a substance having a high electron-trapping property to a material having a high electron-transport property described above, and the layer is capable of adjusting carrier balance by suppressing transfer of electron carriers. Such a structure is very effective in preventing a problem (such as a reduction in element lifetime) caused when electrons pass through the light-emitting layer.

<<Electron-injection Layer>>

The electron-injection layer 119 has a function of reducing a barrier for electron injection from the electrode 102 to promote electron injection and can be formed using a Group 1 metal or a Group 2 metal, or an oxide, a halide, or a carbonate of any of the metals, for example. Alternatively, a composite material containing an electron-transport material (described above) and a material having a property of donating electrons to the electron-transport material can also be used. As the material having an electron-donating property, a Group 1 metal, a Group 2 metal, an oxide of any of the metals, or the like can be given. Specifically, an alkali metal, an alkaline earth metal, or a compound thereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride (CaF₂), or lithium oxide (LiO_(x)), can be used. Alternatively, a rare earth metal compound like erbium fluoride (ErF₃) can be used. Electride may also be used for the electron-injection layer 119. Examples of the electride include a substance in which electrons are added at high concentration to calcium oxide-aluminum oxide. The electron-injection layer 119 can be formed using the substance that can be used for the electron-transport layer 118.

A composite material in which an organic compound and an electron donor (donor) are mixed may also be used for the electron-injection layer 119. Such a composite material is excellent in an electron-injection property and an electron-transport property because electrons are generated in the organic compound by the electron donor. In this case, the organic compound is preferably a material that is excellent in transporting the generated electrons. Specifically, the above-listed substances for forming the electron-transport layer 118 (e.g., the metal complexes and heteroaromatic compounds) can be used, for example. As the electron donor, a substance showing an electron-donating property with respect to the organic compound may be used. Specifically, an alkali metal, an alkaline earth metal, and a rare earth metal are preferable, and lithium, cesium, magnesium, calcium, erbium, and ytterbium are given. In addition, an alkali metal oxide or an alkaline earth metal oxide is preferable, and lithium oxide, calcium oxide, barium oxide, and the like are given. A Lewis base such as magnesium oxide can also be used. An organic compound such as tetrathiafulvalene (abbreviation: TTF) can also be used.

Note that the light-emitting layer, the hole-injection layer, the hole-transport layer, the electron-transport layer, and the electron-injection layer described above can each be formed by an evaporation method (including a vacuum evaporation method), an inkjet method, a coating method, a gravure printing method, or the like. Besides the above-mentioned materials, an inorganic compound such as a quantum dot or a high molecular compound (e.g., an oligomer, a dendrimer, or a polymer) may be used in the light-emitting layer, the hole-injection layer, the hole-transport layer, the electron-transport layer, and the electron-injection layer.

The quantum dot may be a colloidal quantum dot, an alloyed quantum dot, a core-shell quantum dot, or a core quantum dot, for example. The quantum dot containing elements belonging to Groups 2 and 16, elements belonging to Groups 13 and 15, elements belonging to Groups 13 and 17, elements belonging to Groups 11 and 17, or elements belonging to Groups 14 and 15 may be used. Alternatively, the quantum dot containing an element such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P), indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), or aluminum (Al) may be used.

<<Substrate>>

A light-emitting element in one embodiment of the present invention may be formed over a substrate of glass, plastic, or the like. As the way of stacking layers over the substrate, layers may be sequentially stacked from the electrode 101 side or sequentially stacked from the electrode 102 side.

For the substrate over which the light-emitting element of one embodiment of the present invention can be formed, glass, quartz, plastic, or the like can be used, for example. Alternatively, a flexible substrate can be used. The flexible substrate means a substrate that can be bent, such as a plastic substrate made of polycarbonate or polyarylate, for example. Alternatively, a film, an inorganic vapor deposition film, or the like can be used. Another material may be used as long as the substrate functions as a support in a manufacturing process of the light-emitting element or an optical element or as long as it has a function of protecting the light-emitting element or an optical element.

In this specification and the like, a light-emitting element can be formed using any of a variety of substrates, for example. The type of a substrate is not limited particularly. Examples of the substrate include a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, a base material film, and the like. As an example of a glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a soda lime glass substrate, and the like can be given. Examples of the flexible substrate, the attachment film, the base material film, and the like are substrates of plastics typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Another example is a resin such as acrylic. Furthermore, polypropylene, polyester, polyvinyl fluoride, and polyvinyl chloride can be given as examples. Other examples are polyamide, polyimide, aramid, epoxy, an inorganic vapor deposition film, paper, and the like.

Alternatively, a flexible substrate may be used as the substrate such that the light-emitting element is provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate and the light-emitting element. The separation layer can be used when part or the whole of a light-emitting element formed over the separation layer is separated from the substrate and transferred onto another substrate. In such a case, the light-emitting element can be transferred to a substrate having low heat resistance or a flexible substrate as well. For the above separation layer, a stack including inorganic films, which are a tungsten film and a silicon oxide film, or a structure in which a resin film of polyimide or the like is formed over a substrate can be used, for example.

In other words, after the light-emitting element is formed using a substrate, the light-emitting element may be transferred to another substrate. Example of the substrate to which the light-emitting element is transferred are, in addition to the above substrates, a cellophane substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), and the like), a leather substrate, a rubber substrate, and the like. When such a substrate is used, a light-emitting element with high durability, high heat resistance, reduced weight, or reduced thickness can be formed.

The light-emitting element 250 may be formed over an electrode electrically connected to a field-effect transistor (FET), for example, which is formed over any of the above-described substrates. Accordingly, an active matrix display device in which the FET controls the driving of the light-emitting element can be manufactured.

In this embodiment, one embodiment of the present invention has been described. Other embodiments of the present invention are described in the other embodiments. One embodiment of the present invention is not limited to the above-described examples. In one embodiment of the present invention, an example where a light-emitting element contains a fluorescent material and a host material and the minimum value of a distance between centroids of the fluorescent material and the host material is 0.7 nm or more and 5 nm or less has been described; however, one embodiment of the present invention is not limited to this example. Depending on circumstances or conditions, in one embodiment of the present invention, the minimum value of the distance between centroids of the fluorescent material and the host material may be less than 0.7 nm. Alternatively, the minimum value of the distance between centroids of the fluorescent material and the host material may be greater than 5 nm. In addition, in one embodiment of the present invention, an example where a host material contains a first organic compound and a second organic compound has been described; however, one embodiment of the present invention is not limited to this example. Depending on circumstances or conditions, in one embodiment of the present invention, the host material does not necessarily contain the first organic compound or the second organic compound. In addition, in one embodiment of the present invention, an example where a fluorescent material has a fluorescence quantum yield of 70% or higher has been described; however, one embodiment of the present invention is not limited to this example. Depending on circumstances or conditions, in one embodiment of the present invention, the fluorescent material may have a fluorescence quantum yield of lower than 70%. In addition, in one embodiment of the present invention, an example where an exciplex contains a delayed fluorescence material whose fluorescence lifetime is 10 ns or longer and 50 μs or shorter has been described; however, one embodiment of the present invention is not limited to this example. Depending on circumstances or conditions, in one embodiment of the present invention, the exciplex may contain a delayed fluorescence material whose fluorescence lifetime is shorter than 10 ns. Alternatively, the exciplex may contain a delayed fluorescence material whose fluorescence lifetime is longer than 50 μs. In addition, in one embodiment of the present invention, an example where a fluorescent material includes at least two alkyl groups each having 2 or more carbon atoms, an example where a fluorescent material includes at least two branched alkyl groups each having 3 to 10 carbon atoms, an example where a fluorescent material includes at least two cyclic hydrocarbon groups each having 3 to 10 carbon atoms or at least two bridged cyclic hydrocarbon groups each having 3 to 10 carbon atoms, or an example where a fluorescent material includes condensed aromatic hydrocarbon having 3 to 12 carbon atoms has been described; however, one embodiment of the present invention is not limited to these examples. Depending on circumstances or conditions, in one embodiment of the present invention, a fluorescent material does not necessarily include at least two alkyl groups each having 2 or more carbon atoms. Alternatively, a fluorescent material does not necessarily include at least two branched alkyl groups each having 3 to 10 carbon atoms. Alternatively, a fluorescent material does not necessarily include at least two cyclic hydrocarbon groups each having 3 to 10 carbon atoms or at least two bridged cyclic hydrocarbon groups each having 3 to 10 carbon atoms. Alternatively, a fluorescent material does not necessarily include condensed aromatic hydrocarbon having 3 to 12 carbon atoms.

The structure described above in this embodiment can be combined with any of the structures described in the other embodiments as appropriate.

(Embodiment 2)

In this embodiment, light-emitting elements having structures different from that described in Embodiment 1 and light emission mechanisms of the light-emitting elements are described below with reference to FIGS. 6A to 6C and FIGS. 7A to 7C. In FIGS. 6A to 6C and FIGS. 7A to 7C, a portion having a function similar to that in FIG. 1A is represented by the same hatch pattern as in FIG. 1A and not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of the portions is omitted in some cases.

<Structure Example 1 of Light-emitting Element>

FIG. 6A is a schematic cross-sectional view of a light-emitting element 260.

The light-emitting element 260 illustrated in FIG. 6A includes a plurality of light-emitting units (a light-emitting unit 106 and a light-emitting unit 108 in FIG. 6A) between a pair of electrodes (the electrode 101 and the electrode 102). One light-emitting unit has the same structure as the EL layer 100 illustrated in FIG. 1A. That is, the light-emitting element 250 in FIG. 1A includes one light-emitting unit, while the light-emitting element 260 includes a plurality of light-emitting units. Note that the electrode 101 functions as an anode and the electrode 102 functions as a cathode in the following description of the light-emitting element 260; however, the functions may be interchanged in the light-emitting element 260.

In the light-emitting element 260 illustrated in FIG. 6A, the light-emitting unit 106 and the light-emitting unit 108 are stacked, and a charge-generation layer 115 is provided between the light-emitting unit 106 and the light-emitting unit 108. Note that the light-emitting unit 106 and the light-emitting unit 108 may have the same structure or different structures. For example, it is preferable that the EL layer 100 illustrated in FIG. 1A be used in the light-emitting unit 108.

The light-emitting element 260 includes a light-emitting layer 120 and the light-emitting layer 130. The light-emitting unit 106 includes the hole-injection layer 111, the hole-transport layer 112, an electron-transport layer 113, and an electron-injection layer 114 in addition to the light-emitting layer 120. The light-emitting unit 108 includes a hole-injection layer 116, a hole-transport layer 117, an electron-transport layer 118, and an electron-injection layer 119 in addition to the light-emitting layer 130.

The charge-generation layer 115 may have either a structure in which an acceptor substance that is an electron acceptor is added to a hole-transport material or a structure in which a donor substance that is an electron donor is added to an electron-transport material. Alternatively, both of these structures may be stacked.

In the case where the charge-generation layer 115 contains a composite material of an organic compound and an acceptor substance, the composite material that can be used for the hole-injection layer 111 described in Embodiment 1 may be used for the composite material. As the organic compound, a variety of compounds such as an aromatic amine compound, a carbazole compound, an aromatic hydrocarbon, and a high molecular compound (such as an oligomer, a dendrimer, or a polymer) can be used. A substance having a hole mobility of 1×10⁻⁶ cm²/Vs or higher is preferably used as the organic compound. Note that any other material may be used as long as it has a property of transporting more holes than electrons. Since the composite material of an organic compound and an acceptor substance has excellent carrier-injection and carrier-transport properties, low-voltage driving or low-current driving can be realized. Note that when a surface of a light-emitting unit on the anode side is in contact with the charge-generation layer 115 like the light-emitting unit 108, the charge-generation layer 115 can also serve as a hole-injection layer or a hole-transport layer of the light-emitting unit; thus, a hole-injection layer or a hole-transport layer need not be included in the light-emitting unit.

The charge-generation layer 115 may have a stacked structure of a layer containing the composite material of an organic compound and an acceptor substance and a layer containing another material. For example, the charge-generation layer 115 may be formed using a combination of a layer containing the composite material of an organic compound and an acceptor substance with a layer containing one compound selected from among electron-donating materials and a compound having a high electron-transport property. Furthermore, the charge-generation layer 115 may be formed using a combination of a layer containing the composite material of an organic compound and an acceptor substance with a layer containing a transparent conductive material.

The charge-generation layer 115 provided between the light-emitting unit 106 and the light-emitting unit 108 may have any structure as long as electrons can be injected to the light-emitting unit on one side and holes can be injected into the light-emitting unit on the other side when a voltage is applied between the electrode 101 and the electrode 102. For example, in FIG. 6A, the charge-generation layer 115 injects electrons into the light-emitting unit 106 and holes into the light-emitting unit 108 when a voltage is applied such that the potential of the electrode 101 is higher than that of the electrode 102.

Note that in terms of light extraction efficiency, the charge-generation layer 115 preferably has a visible light transmittance (specifically, a visible light transmittance of higher than or equal to 40%). The charge-generation layer 115 functions even if it has lower conductivity than the pair of electrodes (the electrodes 101 and 102). In the case where the conductivity of the charge-generation layer 115 is as high as those of the pair of electrodes, carriers generated in the charge-generation layer 115 flow toward the film surface direction, so that light is emitted in a region where the electrode 101 and the electrode 102 do not overlap, in some cases. To suppress such a defect, the charge-generation layer 115 is preferably formed using a material whose conductivity is lower than those of the pair of electrodes.

Note that forming the charge-generation layer 115 by using any of the above materials can suppress an increase in drive voltage caused by the stack of the light-emitting layers.

The light-emitting element having two light-emitting units is described with reference to FIG. 6A; however, a similar structure can be applied to a light-emitting element in which three or more light-emitting units are stacked. With a plurality of light-emitting units partitioned by the charge-generation layer between a pair of electrodes as in the light-emitting element 260, it is possible to provide a light-emitting element which can emit light with high luminance with the current density kept low and has a long lifetime. A light-emitting element with low power consumption can be provided.

When the structure of the EL layer 100 illustrated in FIG. 1A is used for at least one of the plurality of units, a light-emitting element with high emission efficiency can be provided.

It is preferable that the light-emitting layer 130 included in the light-emitting unit 108 have the structure described in Embodiment 1. Thus, the light-emitting element 260 contains a fluorescent material as a light-emitting material and has high luminous efficiency, which is preferable.

Furthermore, the light-emitting layer 120 included in the light-emitting unit 108 contains a host material 121 and a guest material 122 as illustrated in FIG. 6B. Note that the guest material 122 is described below as a fluorescent material.

<Light Emission Mechanism of Light-emitting Layer 120>

The light emission mechanism of the light-emitting layer 120 is described below.

By recombination of the electrons and holes injected from the pair of electrodes (the electrode 101 and the electrode 102) or the charge-generation layer in the light-emitting layer 120, excitons are formed. Because the amount of the host material 121 is larger than that of the guest material 122, the host material 121 is brought into an excited state by the exciton generation.

Note that the term “exciton” refers to a carrier (electron and hole) pair. Since excitons have energy, a material where excitons are generated is brought into an excited state.

In the case where the formed excited state of the host material 121 is a singlet excited state, singlet excited energy transfers from the S1 level of the host material 121 to the S1 level of the guest material 122, thereby forming the singlet excited state of the guest material 122.

Since the guest material 122 is a fluorescent material, when a singlet excited state is formed in the guest material 122, the guest material 122 readily emits light. To obtain high emission efficiency in this case, the fluorescence quantum yield of the guest material 122 is preferably high. The same can apply to a case where a singlet excited state is formed by recombination of carriers in the guest material 122.

Next, a case where recombination of carriers forms a triplet excited state of the host material 121 is described. The correlation of energy levels of the host material 121 and the guest material 122 in this case is shown in FIG. 6C. The following explains what terms and signs in FIG. 6C represent. Note that because it is preferable that the T1 level of the host material 121 be lower than the T1 level of the guest material 122, FIG. 6C shows this preferable case. However, the T1 level of the host material 121 may be higher than the T1 level of the guest material 122.

Host (121): the host material 121;

Guest (122): the guest material 122 (the fluorescent material);

S_(FH): the S1 level the host material 121;

T_(FH): the T1 level of the host material 121;

S_(FG): the S1 level of the guest material 122 (the fluorescent material); and

T_(FG): the T1 level of the guest material 122 (the fluorescent material).

As illustrated in FIG. 6C, adjacent triplet excitons formed by carrier recombination result in a reaction in which one of the triplet excitons is converted into a singlet exciton having energy of the S1 level of the host material 121 (S_(FH)), that is, triplet-triplet annihilation (TTA) (see TTA in FIG. 6C). The singlet excited energy of the host material 121 is transferred from S_(FH) to the S1 level of the guest material 122 (S_(FG)) having a lower energy than S_(FH) (see Route E₁ in FIG. 6C), and a singlet excited state of the guest material 122 is formed, whereby the guest material 122 emits light.

Note that in the case where the density of triplet excitons in the light-emitting layer 120 is sufficiently high (e.g., 1×10⁻¹² cm⁻³ or more), only the reaction of two triplet excitons close to each other can be considered whereas deactivation of a single triplet exciton can be ignored.

In the case where a triplet excited state of the guest material 122 is formed by carrier recombination, the triplet excited state of the guest material 122 is thermally deactivated and is difficult to use for light emission. However, in the case where the T1 level of the host material 121 (T_(FH)) is lower than the T1 level of the guest material 122 (T_(FG)), the triplet excited energy of the guest material 122 can be transferred from the T1 level of the guest material 122 (T_(FG)) to the T1 level of the host material 121 (T_(FH)) (see Route E₂ in FIG. 6C) and then is utilized for TTA.

In other words, the host material 121 preferably has a function of converting triplet excited energy into singlet excited energy by causing TTA, so that the triplet excited energy generated in the light-emitting layer 120 can be partly converted into singlet excited energy by TTA in the host material 121. The singlet excited energy can be transferred to the guest material 122 and extracted as fluorescence. In order to achieve this, the S1 level of the host material 121 (S_(FH)) is preferably higher than the S1 level of the guest material 122 (S_(FG)). In addition, the T1 level of the host material 121 (T_(FH)) is preferably lower than the T1 level of the guest material 122 (T_(FG)).

Note that particularly in the case where the T1 level of the guest material 122 (T_(FG)) is lower than the T1 level of the host material 121 (T_(FH)), the weight ratio of the guest material 122 to the host material 121 is preferably low. Specifically, the weight ratio of the guest material 122 to the host material 121 is preferably greater than 0 and less than or equal to 0.05, in which case, the probability of carrier recombination in the guest material 122 can be reduced. In addition, the probability of energy transfer from the T1 level of the host material 121 (T_(FH)) to the T1 level of the guest material 122 (T_(FG)) can be reduced.

Note that the host material 121 may be composed of a single compound or a plurality of compounds.

Note that in each of the above-described structures, the guest materials (fluorescent materials) used in the light-emitting unit 106 and the light-emitting unit 108 may be the same or different. In the case where the same guest material is used for the light-emitting unit 106 and the light-emitting unit 108, the light-emitting element 260 can exhibit high emission luminance at a small current value, which is preferable. In the case where different guest materials are used for the light-emitting unit 106 and the light-emitting unit 108, the light-emitting element 260 can exhibit multi-color light emission, which is preferable. It is particularly favorable to select the guest materials so that white light emission with high color rendering properties or light emission of at least red, green, and blue can be obtained.

<Structure Example 2 of Light-emitting Element>

FIG. 7A is a schematic cross-sectional view of a light-emitting element 262.

The light-emitting element 262 illustrated in FIG. 7A includes, like the light-emitting element 260 described above, a plurality of light-emitting units (a light-emitting unit 106 and a light-emitting unit 108 in FIG. 7A) between a pair of electrodes (the electrode 101 and the electrode 102). One light-emitting unit has the same structure as the EL layer 100 illustrated in FIG. 1A. Note that the light-emitting unit 106 and the light-emitting unit 108 may have the same structure or different structures.

In the light-emitting element 262 illustrated in FIG. 7A, the light-emitting unit 106 and the light-emitting unit 108 are stacked, and a charge-generation layer 115 is provided between the light-emitting unit 106 and the light-emitting unit 108. For example, it is preferable that the EL layer 100 illustrated in FIG. 1A be used in the light-emitting unit 106.

The light-emitting element 262 includes the light-emitting layer 130 and a light-emitting layer 140. The light-emitting unit 106 includes the hole-injection layer 111, the hole-transport layer 112, the electron-transport layer 113, and the electron-injection layer 114 in addition to the light-emitting layer 130. The light-emitting unit 108 includes the hole-injection layer 116, the hole-transport layer 117, the electron-transport layer 118, and the electron-injection layer 119 in addition to the light-emitting layer 140.

In addition, the light-emitting layer of the light-emitting unit 108 preferably contains a phosphorescent material. That is, it is preferable that the light-emitting layer 130 included in the light-emitting unit 106 have the structure described in Embodiment 1 and the light-emitting layer 140 included in the light-emitting unit 108 contain a phosphorescent material. A structure example of the light-emitting element 262 in this case is described below.

Furthermore, the light-emitting layer 140 included in the light-emitting unit 108 contains a host material 141 and a guest material 142 as illustrated in FIG. 7B. The host material 141 contains an organic compound 141_1 and an organic compound 141_2. Note that the guest material 142 included in the light-emitting layer 140 is described below as a phosphorescent material.

<Light Emission Mechanism of Light-emitting Layer 140>

Next, the light emission mechanism of the light-emitting layer 140 is described.

The organic compound 141_1 and the organic compound 141_2 which are included in the light-emitting layer 140 form an exciplex.

It is acceptable as long as the combination of the organic compound 141_1 and the organic compound 141_2 can form an exciplex in the light-emitting layer 140, and it is preferred that one organic compound have a hole-transport property and the other organic compound have an electron-transport property.

FIG. 7C illustrates the correlation of energy levels of the organic compound 141_1, the organic compound 141_2, and the guest material 142 in the light-emitting layer 140. The following explains what terms and signs in FIG. 7C represent:

Host (141_1): the organic compound 141_1 (host material);

Host (141_2): the organic compound 141_2 (host material);

Guest (142): the guest material 142 (phosphorescent material);

S_(PH): the S1 level of the organic compound 141_1 (host material);

T_(PH): the T1 level of the organic compound 141_1 (host material);

T_(PG): the T1 level of the guest material 142 (phosphorescent material);

S_(PE): the S1 level of the exciplex; and

T_(PE): the T1 level of the exciplex.

The level (S_(PE)) of the lowest singlet excited state of the exciplex formed by the organic compounds 141_1 and 141_2 and the level (T_(PE)) of the lowest triplet excited state of the exciplex are close to each other (see Route C in FIG. 7C).

Both energies of S_(PE) and T_(PE) of the exciplex are then transferred to the level of the lowest triplet excited state of the guest material 142 (phosphorescent material); thus, light emission is obtained (see Route D in FIG. 7C).

The above-described processes through Route C and Route D may be referred to as exciplex-triplet energy transfer (ExTET) in this specification and the like.

When one of the organic compounds 141_1 and 141_2 receives holes and the other receives electrons, the exciplex is formed. Alternatively, when one compound is brought into an excited state, the one interacts with the other compound to form the exciplex. Therefore, most excitons in the light-emitting layer 140 exist as exciplexes. The band gap of the exciplex is narrower than that of each of the organic compounds 141_1 and 141_2; therefore, an excited state can be formed with lower excitation energy. Thus, the formation of the exciplex can lower the drive voltage of the light-emitting element.

When the light-emitting layer 140 has the above structure, light emission from the guest material 142 (phosphorescent material) of the light-emitting layer 140 can be efficiently obtained.

Note that light emitted from the light-emitting layer 130 preferably has a peak on the shorter wavelength side than light emitted from the light-emitting layer 140. Since the luminance of a light-emitting element using a phosphorescent material emitting light with a short wavelength tends to be degraded quickly, fluorescence with a short wavelength is employed so that a light-emitting element with less degradation of luminance can be provided.

Furthermore, the light-emitting layer 130 and the light-emitting layer 140 may be made to emit light with different emission wavelengths, so that the light-emitting element can be a multicolor light-emitting element. In that case, the emission spectrum of the light-emitting element is formed by combining light having different emission peaks, and thus has at least two peaks.

The above structure is also suitable for obtaining white light emission. When the light-emitting layer 130 and the light-emitting layer 140 emit light of complementary colors, white light emission can be obtained.

In addition, white light emission with a high color rendering property that is formed of three primary colors or four or more colors can be obtained by using a plurality of light-emitting materials emitting light with different wavelengths for one of the light-emitting layers 130 and 140 or both. In that case, one of the light-emitting layers 130 and 140 or both may be divided into layers and each of the divided layers may contain a light-emitting material different from the others.

<Material that can be Used in Light-emitting Layers>

Next, materials that can be used in the light-emitting layers 120, 130, and 140 are described.

<<Material that can be Used in Light-emitting Layer 130>>

As a material that can be used in the light-emitting layer 130, a material that can be used in the light-emitting layer 130 in Embodiment 1 may be used. Thus, a light-emitting element with high generation efficiency of a singlet excited state and high emission efficiency can be fabricated.

<<Material that can be Used in Light-emitting Layer 120>>

In the light-emitting layer 120, the host material 121 is present in the largest proportion by weight, and the guest material 122 (the fluorescent material) is dispersed in the host material 121. The S1 level of the host material 121 is preferably higher than the S1 level of the guest material 122 (the fluorescent material) while the T1 level of the host material 121 is preferably lower than the T1 level of the guest material 122 (the fluorescent material).

In the light-emitting layer 120, as the guest material 122, any of the following materials can be used.

The examples include 5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation: PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine (abbreviation: PAPP2BPy), N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6FLPAPrn), N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine (abbreviation: 1,6mMemFLPAPrn), N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine (abbreviation: YGA2S), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), 4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine (abbreviation: 2YGAPPA), N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene (abbreviation: TBP), 4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation: PCBAPA), N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA), N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: 2PCAPPA), N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPPA), N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine (abbreviation: DBC1), coumarin 30, N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCABPhA), N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPAPA), N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine (abbreviation: 2DPABPhA), 9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine (abbreviation: 2YGABPhA), N,N, 9-triphenylanthracen-9-amine (abbreviation: DPhAPhA), coumarin 6, coumarin 545T, N,N′-diphenylquinacridone (abbreviation: DPQd), rubrene, 5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT), 2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile (abbreviation: DCM1), 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCM2), N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation: p-mPhTD), 7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine (abbreviation: p-mPhAFD), 2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTI), 2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: DCJTB), 2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile (abbreviation: BisDCM), 2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: BisDCJTM), and 5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd:1′,2′,3′-lm]perylene.

Although there is no particular limitation on a material that can be used as the host material 121 in the light-emitting layer 120, any of the following materials can be used, for example: metal complexes such as tris(8-quinolinolato)aluminum(III) (abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq), bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), and bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); heterocyclic compounds such as 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation: PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene (abbreviation: OXD-7), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (abbreviation: TAZ), 2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole) (abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen), bathocuproine (abbreviation: BCP), 2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation: NBphen), and 9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation: CO11); and aromatic amine compounds such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB or α-NPD), N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (abbreviation: TPD), and 4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl (abbreviation: BSPB). In addition, condensed polycyclic aromatic compounds such as anthracene derivatives, phenanthrene derivatives, pyrene derivatives, chrysene derivatives, and dibenzo[g,p]chrysene derivatives can be given, and specific examples are 9,10-diphenylanthracene (abbreviation: DPAnth), N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: CzAlPA), 4-(10-phenyl-9-anthryl)triphenylamine (abbreviation: DPhPA), 4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine (abbreviation: YGAPA), N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine (abbreviation: PCAPA), N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine (abbreviation: PCAPBA), N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine (abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene, N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetramine (abbreviation: DBC1), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), 3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA), 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA), 9,9′-bianthryl (abbreviation: BANT), 9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS), 9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), 1,3,5-tri(1-pyrenyl)benzene (abbreviation: TPB3), and the like. One or more substances having a wider energy gap than the guest material 122 is preferably selected from these substances and known substances.

The light-emitting layer 120 can have a structure in which two or more layers are stacked. For example, in the case where the light-emitting layer 120 is formed by stacking a first light-emitting layer and a second light-emitting layer in this order from the hole-transport layer side, the first light-emitting layer is formed using a substance having a hole-transport property as the host material and the second light-emitting layer is formed using a substance having an electron-transport property as the host material.

In the light-emitting layer 120, the host material 121 may be composed of one kind of compound or a plurality of compounds. Alternatively, the light-emitting layer 120 may contain a material other than the host material 121 and the guest material 122.

<<Material that can be Used in Light-emitting Layer 140>>

In the light-emitting layer 140, the host material 141 exists in the largest proportion in weight ratio, and the guest material 142 (phosphorescent material) is dispersed in the host material 141. The T1 levels of the host materials 141 (organic compounds 141_1 and 141_2) of the light-emitting layer 140 are preferably higher than the T1 level of the guest material (guest material 142) of the light-emitting layer 140.

Examples of the organic compound 141_1 include a zinc- or aluminum-based metal complex, an oxadiazole derivative, a triazole derivative, a benzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a pyrimidine derivative, a triazine derivative, a pyridine derivative, a bipyridine derivative, a phenanthroline derivative, and the like. Other examples are an aromatic amine, a carbazole derivative, and the like. Specifically, the electron-transport material and the hole-transport material described in Embodiment 1 can be used.

As the organic compound 141_2, a substance which can form an exciplex together with the organic compound 141_1 is used. Specifically, the electron-transport material and the hole-transport material described in Embodiment 1 can be used. In that case, it is preferable that the organic compound 141_1, the organic compound 141_2, and the guest material 142 (phosphorescent material) be selected such that the emission peak of the exciplex formed by the organic compound 141_1 and the organic compound 141_2 overlaps with an absorption band, specifically an absorption band on the longest wavelength side, of a triplet metal to ligand charge transfer (MLCT) transition of the guest material 142 (phosphorescent material). This makes it possible to provide a light-emitting element with drastically improved emission efficiency. Note that in the case where a thermally activated delayed fluorescent material is used instead of the phosphorescent material, it is preferable that the absorption band on the longest wavelength side be a singlet absorption band.

As the guest material 142 (phosphorescent material), an iridium-, rhodium-, or platinum-based organometallic complex or metal complex can be used; in particular, an organoiridium complex such as an iridium-based ortho-metalated complex is preferable. As an ortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, an imidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazine ligand, an isoquinoline ligand, and the like can be given. As the metal complex, a platinum complex having a porphyrin ligand and the like can be given.

Examples of the substance that has an emission peak in the blue or green wavelength range include organometallic iridium complexes having a 4H-triazole skeleton, such as tris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III) (abbreviation: Ir(mpptz-dmp)₃), tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Mptz)₃), tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(iPrptz-3b)₃), and tris[3-(5-biphenyl)-5-isopropyl-4-phenyl-4H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(iPr5btz)₃); organometallic iridium complexes having a 1H-triazole skeleton, such as tris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III) (abbreviation: Ir(Mptz1-mp)₃) and tris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III) (abbreviation: Ir(Prptz1-Me)₃); organometallic iridium complexes having an imidazole skeleton, such as fac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III) (abbreviation: Ir(iPrpmi)₃) and tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III) (abbreviation: Ir(dmpimpt-Me)₃); and organometallic iridium complexes in which a phenylpyridine derivative having an electron-withdrawing group is a ligand, such as bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) tetrakis(1-pyrazolyl)borate (abbreviation: FIr6), bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²]iridium(III) picolinate (abbreviation: FIrpic), bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), and bis[2-(4′,6′-difluorophenyl)pyridinato-N,C²]iridium(III) acetylacetonate (abbreviation: FIr(acac)). Among the materials given above, the organometallic iridium complexes having a 4H-triazole skeleton have high reliability and high emission efficiency and are thus especially preferable.

Examples of the substance that has an emission peak in the green or yellow wavelength range include organometallic iridium complexes having a pyrimidine skeleton, such as tris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)₃), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)₃), (acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(mppm)₂(acac)), (acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III) (abbreviation: Ir(tBuppm)₂(acac)), (acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III) (abbreviation: Ir(nbppm)₂(acac)), (acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III) (abbreviation: Ir(mpmppm)₂(acac)), (acetylacetonato)bis {4,6-dimethyl-2-[6-(2,6-dimethylphenyl)-4-pyrimidinyl-κN3]phenyl-1-κC}iridium(III) (abbreviation: Ir(dmppm-dmp)₂(acac)), (acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III) (abbreviation: Ir(dppm)₂(acac)); organometallic iridium complexes having a pyrazine skeleton, such as (acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-Me)₂(acac)) and (acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III) (abbreviation: Ir(mppr-iPr)₂(acac)); organometallic iridium complexes having a pyridine skeleton, such as tris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃), bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate (abbreviation: Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III) acetylacetonate (abbreviation: Ir(bzq)₂(acac)), tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃), tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃), and bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate (abbreviation: Ir(pq)₂(acac)); organometallic iridium complexes such as bis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(dpo)₂(acac)), bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)acetylacetonate (abbreviation: Ir(p-PF-ph)₂(acac)), and bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(bt)₂(acac)); and a rare earth metal complex such as tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation: Tb(acac)₃(Phen)). Among the materials given above, the organometallic iridium complexes having a pyrimidine skeleton have distinctively high reliability and emission efficiency and are thus particularly preferable.

Examples of the substance that has an emission peak in the yellow or red wavelength range include organometallic iridium complexes having a pyrimidine skeleton, such as (diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III) (abbreviation: Ir(5mdppm)₂(dibm)), bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(5mdppm)₂(dpm)), and bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(dlnpm)₂(dpm)); organometallic iridium complexes having a pyrazine skeleton, such as (acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III) (abbreviation: Ir(tppr)₂(acac)), bis(2,3,5-triphenylpyrazinato) (dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr)₂(dpm)), and (acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III) (abbreviation: Ir(Fdpq)₂(acac)); organometallic iridium complexes having a pyridine skeleton, such as tris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(piq)₃) and bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(piq)₂(acac)); a platinum complex such as 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II) (abbreviation: PtOEP); and rare earth metal complexes such as tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III) (abbreviation: Eu(DBM)₃(Phen)) and tris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)₃(Phen)). Among the materials given above, the organometallic iridium complexes having a pyrimidine skeleton have distinctively high reliability and emission efficiency and are thus particularly preferable. Further, the organometallic iridium complexes having a pyrazine skeleton can provide red light emission with favorable chromaticity.

As the light-emitting material included in the light-emitting layer 140, any material can be used as long as the material can convert the triplet excitation energy into light emission. As an example of the material that can convert the triplet excitation energy into light emission, a thermally activated delayed fluorescent (TADF) material can be given in addition to a phosphorescent material. Therefore, it is acceptable that the “phosphorescent material” in the description is replaced with the “thermally activated delayed fluorescence material”. Note that the thermally activated delayed fluorescence material is a material having a small difference between the triplet excited energy level and the singlet excited energy level and a function of converting triplet excited energy into singlet excited energy by reverse intersystem crossing. Thus, the TADF material can up-convert a triplet excited state into a singlet excited state (i.e., reverse intersystem crossing is possible) using a little thermal energy and efficiently exhibit light emission (fluorescence) from the singlet excited state. The TADF is efficiently obtained under the condition where the difference in energy between the triplet excited energy level and the singlet excited energy level is preferably greater than 0 eV and less than or equal to 0.2 eV, further preferably greater than 0 eV and less than or equal to 0.1 eV.

The material that exhibits thermally activated delayed fluorescence may be a material that can form a singlet excited state by itself from a triplet excited state by reverse intersystem crossing or may be a combination of a plurality of materials which form an exciplex.

In the case where the material exhibiting thermally activated delayed fluorescence is formed of one kind of material, any of the thermally activated delayed fluorescent materials described in Embodiment 1 can be specifically used.

In the case where the thermally activated delayed fluorescent material is used as the host material, it is preferable to use a combination of two kinds of compounds which form an exciplex. In this case, it is particularly preferable to use the above-described combination of a compound which easily accepts electrons and a compound which easily accepts holes, which form an exciplex.

There is no limitation on the emission colors of the light-emitting materials contained in the light-emitting layers 120, 130, and 140, and they may be the same or different. Light emitted from the light-emitting materials is mixed and extracted out of the element; therefore, for example, in the case where their emission colors are complementary colors, the light-emitting element can emit white light. In consideration of the reliability of the light-emitting element, the emission peak wavelength of the light-emitting material included in the light-emitting layer 120 is preferably shorter than that of the light-emitting material included in the light-emitting layer 140.

Note that the light-emitting units 106 and 108 and the charge-generation layer 115 can be formed by an evaporation method (including a vacuum evaporation method), an ink-jet method, a coating method, gravure printing, or the like.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

(Embodiment 3)

In this embodiment, examples of light-emitting elements having structures different from those described in Embodiments 1 and 2 are described below with reference to FIGS. 8A and 8B, FIGS. 9A and 9B, FIGS. 10A to 10C, and FIGS. 11A to 11C.

<Structure Example 1 of Light-emitting Element>

FIGS. 8A and 8B are cross-sectional views each illustrating a light-emitting element of one embodiment of the present invention. In FIGS. 8A and 8B, a portion having a function similar to that in FIG. 1A is represented by the same hatch pattern as in FIG. 1A and not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of the portions is omitted in some cases.

Light-emitting elements 270 a and 270 b in FIGS. 8A and 8B may have a bottom-emission structure in which light is extracted through the substrate 200 or may have a top-emission structure in which light emitted from the light-emitting element is extracted in the direction opposite to the substrate 200. However, one embodiment of the present invention is not limited to this structure, and a light-emitting element having a dual-emission structure in which light emitted from the light-emitting element is extracted in both top and bottom directions of the substrate 200 may be used.

In the case where the light-emitting elements 270 a and 270 b each have a bottom emission structure, the electrode 101 preferably has a function of transmitting light and the electrode 102 preferably has a function of reflecting light. Alternatively, in the case where the light-emitting elements 270 a and 270 b each have a top emission structure, the electrode 101 preferably has a function of reflecting light and the electrode 102 preferably has a function of transmitting light.

The light-emitting elements 270 a and 270 b each include the electrode 101 and the electrode 102 over the substrate 200. Between the electrodes 101 and 102, a light-emitting layer 123B, a light-emitting layer 123G, and a light-emitting layer 123R are provided. The hole-injection layer 111, the hole-transport layer 112, the electron-transport layer 118, and the electron-injection layer 119 are also provided.

The light-emitting element 270 b includes, as part of the electrode 101, a conductive layer 101 a, a conductive layer 101 b over the conductive layer 101 a, and a conductive layer 101 c under the conductive layer 101 a. In other words, the light-emitting element 270 b includes the electrode 101 having a structure in which the conductive layer 101 a is sandwiched between the conductive layer 101 b and the conductive layer 101 c.

In the light-emitting element 270 b, the conductive layer 101 b and the conductive layer 101 c may be formed with different materials or the same material. The electrode 101 preferably has a structure in which the conductive layer 101 a is sandwiched by the layers formed of the same conductive material, in which case patterning by etching can be performed easily.

In the light-emitting element 270 b, the electrode 101 may include one of the conductive layer 101 b and the conductive layer 101 c.

For each of the conductive layers 101 a, 101 b, and 101 c, which are included in the electrode 101, the structure and materials of the electrode 101 or 102 described in Embodiment 1 can be used.

In FIGS. 8A and 8B, a partition wall 145 is provided between a region 221B, a region 221G, and a region 221R, which are sandwiched between the electrode 101 and the electrode 102. The partition wall 145 has an insulating property. The partition wall 145 covers end portions of the electrode 101 and has openings overlapping with the electrode. With the partition wall 145, the electrode 101 provided over the substrate 200 in the regions can be divided into island shapes.

Note that the light-emitting layer 123B and the light-emitting layer 123G may overlap with each other in a region where they overlap with the partition wall 145. The light-emitting layer 123G and the light-emitting layer 123R may overlap with each other in a region where they overlap with the partition wall 145. The light-emitting layer 123R and the light-emitting layer 123B may overlap with each other in a region where they overlap with the partition wall 145.

The partition wall 145 has an insulating property and is formed using an inorganic or organic material. Examples of the inorganic material include silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, and aluminum nitride. Examples of the organic material include photosensitive resin materials such as an acrylic resin and a polyimide resin.

The light-emitting layers 123R, 123G, and 123B preferably contain light-emitting materials having functions of emitting light of different colors. For example, when the light-emitting layer 123R contains a light-emitting material having a function of emitting red, the region 221R emits red light. When the light-emitting layer 123G contains a light-emitting material having a function of emitting green, the region 221G emits green light. When the light-emitting layer 123B contains a light-emitting material having a function of emitting blue, the region 221B emits blue light. The light-emitting element 270 a or 270 b having such a structure is used in a pixel of a display device, whereby a full-color display device can be fabricated. The thicknesses of the light-emitting layers may be the same or different.

Any one or more of the light-emitting layers 123B, 123G, and 123R preferably include the light-emitting layer 130 described in Embodiment 1, in which case a light-emitting element with high emission efficiency can be fabricated.

One or more of the light-emitting layers 123B, 123G, and 123R may include two or more stacked layers.

When at least one light-emitting layer includes the light-emitting layer described in Embodiment 1 as described above and the light-emitting element 270 a or 270 b including the light-emitting layer is used in pixels in a display device, a display device with high emission efficiency can be fabricated. The display device including the light-emitting element 270 a or 270 b can thus have reduced power consumption.

By providing a color filter over the electrode through which light is extracted, the color purity of each of the light-emitting elements 270 a and 270 b can be improved. Therefore, the color purity of a display device including the light-emitting element 270 a or 270 b can be improved.

By providing a polarizing plate over the electrode through which light is extracted, the reflection of external light by each of the light-emitting elements 270 a and 270 b can be reduced. Therefore, the contrast ratio of a display device including the light-emitting element 270 a or 270 b can be improved.

For the other components of the light-emitting elements 270 a and 270 b, the components of the light-emitting element in Embodiment 1 may be referred to.

<Structure Example 2 of Light-emitting Element>

Next, structure examples different from the light-emitting elements illustrated in FIGS. 8A and 8B will be described below with reference to FIGS. 9A and 9B.

FIGS. 9A and 9B are cross-sectional views of a light-emitting element of one embodiment of the present invention. In FIGS. 9A and 9B, a portion having a function similar to that in FIGS. 8A and 8B is represented by the same hatch pattern as in FIGS. 8A and 8B and not especially denoted by a reference numeral in some cases. In addition, common reference numerals are used for portions having similar functions, and a detailed description of such portions is not repeated in some cases.

FIGS. 9A and 9B illustrate structure examples of a light-emitting element including the light-emitting layer between a pair of electrodes. A light-emitting element 272 a illustrated in FIG. 9A has a top-emission structure in which light is extracted in a direction opposite to the substrate 200, and a light-emitting element 272 b illustrated in FIG. 9B has a bottom-emission structure in which light is extracted to the substrate 200 side. However, one embodiment of the present invention is not limited to these structures and may have a dual-emission structure in which light emitted from the light-emitting element is extracted in both top and bottom directions with respect to the substrate 200 over which the light-emitting element is formed.

The light-emitting elements 272 a and 272 b each include the electrode 101, the electrode 102, an electrode 103, and an electrode 104 over the substrate 200. At least a light-emitting layer 130 and the charge-generation layer 115 are provided between the electrode 101 and the electrode 102, between the electrode 102 and the electrode 103, and between the electrode 102 and the electrode 104. The hole-injection layer 111, the hole-transport layer 112, a light-emitting layer 150, the electron-transport layer 113, the electron-injection layer 114, the hole-injection layer 116, the hole-transport layer 117, the electron-transport layer 118, and the electron-injection layer 119 are further provided.

The electrode 101 includes a conductive layer 101 a and a conductive layer 101 b over and in contact with the conductive layer 101 a. The electrode 103 includes a conductive layer 103 a and a conductive layer 103 b over and in contact with the conductive layer 103 a. The electrode 104 includes a conductive layer 104 a and a conductive layer 104 b over and in contact with the conductive layer 104 a.

The light-emitting element 272 a illustrated in FIG. 9A and the light-emitting element 272 b illustrated in FIG. 9B each include a partition wall 145 between a region 222B sandwiched between the electrode 101 and the electrode 102, a region 222G sandwiched between the electrode 102 and the electrode 103, and a region 222R sandwiched between the electrode 102 and the electrode 104. The partition wall 145 has an insulating property. The partition wall 145 covers end portions of the electrodes 101, 103, and 104 and has openings overlapping with the electrodes. With the partition wall 145, the electrodes provided over the substrate 200 in the regions can be separated into island shapes.

The light-emitting elements 272 a and 272 b each include a substrate 220 provided with an optical element 224B, an optical element 224G, and an optical element 224R in the direction in which light emitted from the region 222B, light emitted from the region 222G, and light emitted from the region 222R are extracted. The light emitted from each region is emitted outside the light-emitting element through each optical element. In other words, the light from the region 222B, the light from the region 222G, and the light from the region 222R are emitted through the optical element 224B, the optical element 224G, and the optical element 224R, respectively.

The optical elements 224B, 224G, and 224R each have a function of selectively transmitting light of a particular color out of incident light. For example, the light emitted from the region 222B through the optical element 224B is blue light, the light emitted from the region 222G through the optical element 224G is green light, and the light emitted from the region 222R through the optical element 224R is red light.

For example, a coloring layer (also referred to as color filter), a band pass filter, a multilayer filter, or the like can be used for the optical elements 224R, 224G, and 224B. Alternatively, color conversion elements can be used as the optical elements. A color conversion element is an optical element that converts incident light into light having a longer wavelength than the incident light. As the color conversion elements, quantum-dot elements can be favorably used. The usage of the quantum-dot type can increase color reproducibility of the display device.

A plurality of optical elements may also be stacked over each of the optical elements 224R, 224G, and 224B. As another optical element, a circularly polarizing plate, an anti-reflective film, or the like can be provided, for example. A circularly polarizing plate provided on the side where light emitted from the light-emitting element of the display device is extracted can prevent a phenomenon in which light entering from the outside of the display device is reflected inside the display device and returned to the outside. An anti-reflective film can weaken external light reflected by a surface of the display device. This leads to clear observation of light emitted from the display device.

Note that in FIGS. 9A and 9B, blue light (B), green light (G), and red light (R) emitted from the regions through the optical elements are schematically illustrated by arrows of dashed lines.

A light-blocking layer 223 is provided between the optical elements. The light-blocking layer 223 has a function of blocking light emitted from the adjacent regions. Note that a structure without the light-blocking layer 223 may also be employed.

The light-blocking layer 223 has a function of reducing the reflection of external light. The light-blocking layer 223 has a function of preventing mixture of light emitted from an adjacent light-emitting element. As the light-blocking layer 223, a metal, a resin containing black pigment, carbon black, a metal oxide, a composite oxide containing a solid solution of a plurality of metal oxides, or the like can be used.

For the substrate 200 and the substrate 220 provided with the optical elements, the substrate in Embodiment 1 may be referred to.

Furthermore, the light-emitting elements 272 a and 272 b have a microcavity structure.

Light emitted from the light-emitting layer 130 and the light-emitting layer 150 resonates between a pair of electrodes (e.g., the electrode 101 and the electrode 102). The light-emitting layer 130 and the light-emitting layer 150 are formed at such a position as to intensify the light of a desired wavelength among light to be emitted. For example, by adjusting the optical length from a reflective region of the electrode 101 to the light-emitting region of the light-emitting layer 130 and the optical length from a reflective region of the electrode 102 to the light-emitting region of the light-emitting layer 130, the light of a desired wavelength among light emitted from the light-emitting layer 130 can be intensified. By adjusting the optical length from the reflective region of the electrode 101 to the light-emitting region of the light-emitting layer 150 and the optical length from the reflective region of the electrode 102 to the light-emitting region of the light-emitting layer 150, the light of a desired wavelength among light emitted from the light-emitting layer 150 can be intensified. In the case of a light-emitting element in which a plurality of light-emitting layers (here, the light-emitting layers 130 and 150) are stacked, the optical lengths of the light-emitting layers 130 and 150 are preferably optimized.

In each of the light-emitting elements 272 a and 272 b, by adjusting the thicknesses of the conductive layers (the conductive layer 101 b, the conductive layer 103 b, and the conductive layer 104 b) in each region, the light of a desired wavelength among light emitted from the light-emitting layers 130 and 150 can be increased. Note that the thickness of at least one of the hole-injection layer 111 and the hole-transport layer 112 may differ between the regions to increase the light emitted from the light-emitting layers 130 and 150.

For example, in the case where the refractive index of the conductive material having a function of reflecting light in the electrodes 101 to 104 is lower than the refractive index of the light-emitting layer 130 or 150, the thickness of the conductive layer 101 b of the electrode 101 is adjusted so that the optical length between the electrode 101 and the electrode 102 is m_(B)λ_(B)/2 (m_(B) is a natural number and λ_(B) is the wavelength of light intensified in the region 222B). Similarly, the thickness of the conductive layer 103 b of the electrode 103 is adjusted so that the optical length between the electrode 103 and the electrode 102 is m_(G)λ_(G)/2 (m_(G) is a natural number and λ_(G) is the wavelength of light intensified in the region 222G). Furthermore, the thickness of the conductive layer 104 b of the electrode 104 is adjusted so that the optical length between the electrode 104 and the electrode 102 is m_(R)λ_(R)/2 (m_(R) is a natural number and λ_(R) is the wavelength of light intensified in the region 222R).

In the above manner, with the microcavity structure, in which the optical length between the pair of electrodes in the respective regions is adjusted, scattering and absorption of light in the vicinity of the electrodes can be suppressed, resulting in high light extraction efficiency. In the above structure, the conductive layers 101 b, 103 b, and 104 b preferably have a function of transmitting light. The materials of the conductive layers 101 b, 103 b, and 104 b may be the same or different. Each of the conductive layers 101 b, 103 b, and 104 b may have a stacked structure of two or more layers.

Since the light-emitting element 272 a illustrated in FIG. 9A has a top-emission structure, it is preferable that the conductive layer 101 a, the conductive layer 103 a, and the conductive layer 104 a have a function of reflecting light. In addition, it is preferable that the electrode 102 have functions of transmitting light and reflecting light.

Since the light-emitting element 272 b illustrated in FIG. 9B has a bottom-emission structure, it is preferable that the conductive layer 101 a, the conductive layer 103 a, and the conductive layer 104 a have functions of transmitting light and reflecting light. In addition, it is preferable that the electrode 102 have a function of reflecting light.

In each of the light-emitting elements 272 a and 272 b, the conductive layers 101 a, 103 a, and 104 a may be formed of different materials or the same material. When the conductive layers 101 a, 103 a, and 104 a are formed of the same material, manufacturing cost of the light-emitting elements 272 a and 272 b can be reduced. Note that each of the conductive layers 101 a, 103 a, and 104 a may have a stacked structure including two or more layers.

The light-emitting layer 130 in the light-emitting elements 272 a and 272 b preferably has the structure described in Embodiment 1, in which case light-emitting elements with high emission efficiency can be fabricated.

Either or both of the light-emitting layers 130 and 150 may have a stacked structure of two layers, like a light-emitting layer 150 a and a light-emitting layer 150 b. The two light-emitting layers including two kinds of light-emitting materials (a first light-emitting material and a second light-emitting material) for emitting different colors of light enable light emission of a plurality of colors. It is particularly preferable to select the light-emitting materials of the light-emitting layers so that white light can be obtained by combining light emissions from the light-emitting layers 130 and 150.

Either or both of the light-emitting layers 130 and 150 may have a stacked structure of three or more layers, in which a layer not including a light-emitting material may be included.

In the above-described manner, the light-emitting element 272 a or 272 b including the light-emitting layer which has the structure described in Embodiment 1 is used in pixels in a display device, whereby a display device with high emission efficiency can be fabricated. Accordingly, the display device including the light-emitting element 272 a or 272 b can have low power consumption.

For the other components of the light-emitting elements 272 a and 272 b, the components of the light-emitting element 270 a or 270 b or the light-emitting element in Embodiment 1 or 2 may be referred to.

<Fabrication Method of Light-emitting Element>

Next, a method for fabricating a light-emitting element of one embodiment of the present invention is described below with reference to FIGS. 10A to 10C and FIGS. 11A to 11C. Here, a method for fabricating the light-emitting element 272 a illustrated in FIG. 9A is described.

FIGS. 10A to 10C and FIGS. 11A to 11C are cross-sectional views illustrating a method for fabricating the light-emitting element of one embodiment of the present invention.

The method for manufacturing the light-emitting element 272 a described below includes first to seventh steps.

<<First Step>>

In the first step, the electrodes (specifically the conductive layer 101 a of the electrode 101, the conductive layer 103 a of the electrode 103, and the conductive layer 104 a of the electrode 104) of the light-emitting elements are formed over the substrate 200 (see FIG. 10A).

In this embodiment, a conductive layer having a function of reflecting light is formed over the substrate 200 and processed into a desired shape; whereby the conductive layers 101 a, 103 a, and 104 a are formed. As the conductive layer having a function of reflecting light, an alloy film of silver, palladium, and copper (also referred to as an Ag—Pd—Cu film and APC) is used. The conductive layers 101 a, 103 a, and 104 a are preferably formed through a step of processing the same conductive layer, because the manufacturing cost can be reduced.

Note that a plurality of transistors may be formed over the substrate 200 before the first step. The plurality of transistors may be electrically connected to the conductive layers 101 a, 103 a, and 104 a.

<<Second Step>>

In the second step, the transparent conductive layer 101 b having a function of transmitting light is formed over the conductive layer 101 a of the electrode 101, the transparent conductive layer 103 b having a function of transmitting light is formed over the conductive layer 103 a of the electrode 103, and the transparent conductive layer 104 b having a function of transmitting light is formed over the conductive layer 104 a of the electrode 104 (see FIG. 10B).

In this embodiment, the conductive layers 101 b, 103 b, and 104 b each having a function of transmitting light are formed over the conductive layers 101 a, 103 a, and 104 a each having a function of reflecting light, respectively, whereby the electrode 101, the electrode 103, and the electrode 104 are formed. As the conductive layers 101 b, 103 b, and 104 b, ITSO films are used.

The conductive layers 101 b, 103 b, and 104 b having a function of transmitting light may be formed through a plurality of steps. When the conductive layers 101 b, 103 b, and 104 b having a function of transmitting light are formed through a plurality of steps, they can be formed to have thicknesses which enable microcavity structures appropriate in the respective regions.

<<Third Step>>

In the third step, the partition wall 145 that covers end portions of the electrodes of the light-emitting element is formed (see FIG. 10C).

The partition wall 145 includes an opening overlapping with the electrode. The conductive film exposed by the opening functions as the anode of the light-emitting element. As the partition wall 145, a polyimide-based resin is used in this embodiment.

In the first to third steps, since there is no possibility of damaging the EL layer (a layer containing an organic compound), a variety of film formation methods and fine processing technologies can be employed. In this embodiment, a reflective conductive layer is formed by a sputtering method, a pattern is formed over the conductive layer by a lithography method, and then the conductive layer is processed into an island shape by a dry etching method or a wet etching method to form the conductive layer 101 a of the electrode 101, the conductive layer 103 a of the electrode 103, and the conductive layer 104 a of the electrode 104. Then, a transparent conductive film is formed by a sputtering method, a pattern is formed over the transparent conductive film by a lithography method, and then the transparent conductive film is processed into island shapes by a wet etching method to form the electrodes 101, 103, and 104.

<<Fourth Step>>

In the fourth step, the hole-injection layer 111, the hole-transport layer 112, the light-emitting layer 150, the electron-transport layer 113, the electron-injection layer 114, and the charge-generation layer 115 are formed (see FIG. 11A).

The hole-injection layer 111 can be formed by co-evaporating a hole-transport material and a material containing an acceptor substance. Note that a co-evaporation method is an evaporation method in which a plurality of different substances is concurrently vaporized from respective different evaporation sources. The hole-transport layer 112 can be formed by evaporating a hole-transport material.

The light-emitting layer 150 can be formed by evaporating the guest material that emits light of at least one of green, yellow green, yellow, orange, and red. As the guest material, a fluorescent or phosphorescent organic compound can be used. In addition, the light-emitting layer having any of the structures described in Embodiments 1 and 2 is preferably used. The light-emitting layer 150 may have a two-layer structure. In that case, the two light-emitting layers preferably contain light-emitting substances that emit light of different colors.

The electron-transport layer 113 can be formed by evaporating a substance having a high electron-transport property. The electron-injection layer 114 can be formed by evaporating a substance having a high electron-injection property.

The charge-generation layer 115 can be formed by evaporating a material obtained by adding an electron acceptor (acceptor) to a hole-transport material or a material obtained by adding an electron donor (donor) to an electron-transport material.

<<Fifth Step>>

In the fifth step, the hole-injection layer 116, the hole-transport layer 117, the light-emitting layer 130, the electron-transport layer 118, the electron-injection layer 119, and the electrode 102 are formed (see FIG. 11B).

The hole-injection layer 116 can be formed by using a material and a method which are similar to those of the hole-injection layer 111. The hole-transport layer 117 can be formed by using a material and a method which are similar to those of the hole-transport layer 112.

The light-emitting layer 130 can be formed by evaporating the guest material that emits light of at least one color selected from violet, blue, and blue green. As the guest material, a fluorescent organic compound can be used. The fluorescent organic compound may be evaporated alone or the fluorescent organic compound mixed with another material may be evaporated. For example, the fluorescent organic compound may be used as a guest material, and the guest material may be dispersed into a host material having higher excitation energy than the guest material.

The electron-transport layer 118 can be formed by using a material and a method which are similar to those of the electron-transport layer 113. The electron-injection layer 119 can be formed by using a material and a method which are similar to those of the electron-injection layer 114.

The electrode 102 can be formed by stacking a reflective conductive film and a light-transmitting conductive film. The electrode 102 may have a single-layer structure or a stacked-layer structure.

Through the above-described steps, the light-emitting element including the region 222B, the region 222G, and the region 222R over the electrode 101, the electrode 103, and the electrode 104, respectively, are formed over the substrate 200.

<<Sixth Step>>

In the sixth step, the light-blocking layer 223, the optical element 224B, the optical element 224G, and the optical element 224R are formed over the substrate 220 (see FIG. 11C).

As the light-blocking layer 223, a resin film containing black pigment is formed in a desired region. Then, the optical element 224B, the optical element 224G, and the optical element 224R are formed over the substrate 220 and the light-blocking layer 223. As the optical element 224B, a resin film containing blue pigment is formed in a desired region. As the optical element 224G, a resin film containing green pigment is formed in a desired region. As the optical element 224R, a resin film containing red pigment is formed in a desired region.

<<Seventh Step>>

In the seventh step, the light-emitting element formed over the substrate 200 is attached to the light-blocking layer 223, the optical element 224B, the optical element 224G, and the optical element 224R formed over the substrate 220, and sealed with a sealant (not illustrated).

Through the above-described steps, the light-emitting element 272 a illustrated in FIG. 9A can be formed.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

(Embodiment 4)

In this embodiment, a display device of one embodiment of the present invention will be described below with reference to FIGS. 12A and 12B, FIGS. 13A and 13B, FIG. 14, FIGS. 15A and 15B, FIGS. 16A and 16B, FIG. 17, and FIGS. 18A and 18B.

<Structure Example 1 of Display Device>

FIG. 12A is a top view illustrating a display device 600 and FIG. 12B is a cross-sectional view taken along the dashed-dotted line A-B and the dashed-dotted line C-D in FIG. 12A. The display device 600 includes driver circuit portions (a signal line driver circuit portion 601 and a scan line driver circuit portion 603) and a pixel portion 602. Note that the signal line driver circuit portion 601, the scan line driver circuit portion 603, and the pixel portion 602 have a function of controlling light emission of a light-emitting element.

The display device 600 also includes an element substrate 610, a sealing substrate 604, a sealant 605, a region 607 surrounded by the sealant 605, a lead wiring 608, and an FPC 609.

Note that the lead wiring 608 is a wiring for transmitting signals to be input to the signal line driver circuit portion 601 and the scan line driver circuit portion 603 and for receiving a video signal, a clock signal, a start signal, a reset signal, and the like from the FPC 609 serving as an external input terminal. Although only the FPC 609 is illustrated here, the FPC 609 may be provided with a printed wiring board (PWB).

As the signal line driver circuit portion 601, a CMOS circuit in which an n-channel transistor 623 and a p-channel transistor 624 are combined is formed. As the signal line driver circuit portion 601 or the scan line driver circuit portion 603, various types of circuits such as a CMOS circuit, a PMOS circuit, or an NMOS circuit can be used. Although a driver in which a driver circuit portion is formed and a pixel are formed over the same surface of a substrate in the display device of this embodiment, the driver circuit portion is not necessarily formed over the substrate and can be formed outside the substrate.

The pixel portion 602 includes a switching transistor 611, a current control transistor 612, and a lower electrode 613 electrically connected to a drain of the current control transistor 612. Note that a partition wall 614 is formed to cover end portions of the lower electrode 613. As the partition wall 614, for example, a positive type photosensitive acrylic resin film can be used.

In order to obtain favorable coverage by a film which is formed over the partition wall 614, the partition wall 614 is formed to have a curved surface with curvature at its upper or lower end portion. For example, in the case of using a positive photosensitive acrylic as a material of the partition wall 614, it is preferable that only the upper end portion of the partition wall 614 have a curved surface with curvature (the radius of the curvature being 0.2 μm to 3 μm). As the partition wall 614, either a negative photosensitive resin or a positive photosensitive resin can be used.

Note that there is no particular limitation on a structure of each of the transistors (the transistors 611, 612, 623, and 624). For example, a staggered transistor can be used. In addition, there is no particular limitation on the polarity of these transistors. For these transistors, n-channel and p-channel transistors may be used, or either n-channel transistors or p-channel transistors may be used, for example. Furthermore, there is no particular limitation on the crystallinity of a semiconductor film used for these transistors. For example, an amorphous semiconductor film or a crystalline semiconductor film may be used. Examples of a semiconductor material include Group 14 semiconductors (e.g., a semiconductor including silicon), compound semiconductors (including oxide semiconductors), organic semiconductors, and the like. For example, it is preferable to use an oxide semiconductor that has an energy gap of 2 eV or more, preferably 2.5 eV or more and further preferably 3 eV or more, for the transistors, so that the off-state current of the transistors can be reduced. Examples of the oxide semiconductor include an In—Ga oxide and an In-M-Zn oxide (M is aluminum (Al), gallium (Ga), yttrium (Y), zirconium (Zr), lanthanum (La), cerium (Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)).

An EL layer 616 and an upper electrode 617 are formed over the lower electrode 613. Here, the lower electrode 613 functions as an anode and the upper electrode 617 functions as a cathode.

In addition, the EL layer 616 is formed by various methods such as an evaporation method with an evaporation mask, an ink-jet method, or a spin coating method. As another material included in the EL layer 616, a low molecular compound or a high molecular compound (including an oligomer or a dendrimer) may be used.

Note that a light-emitting element 618 is formed with the lower electrode 613, the EL layer 616, and the upper electrode 617. The light-emitting element 618 has any of the structures described in Embodiments 1 to 3. In the case where the pixel portion includes a plurality of light-emitting elements, the pixel portion may include both any of the light-emitting elements described in Embodiments 1 to 3 and a light-emitting element having a different structure.

When the sealing substrate 604 and the element substrate 610 are attached to each other with the sealant 605, the light-emitting element 618 is provided in the region 607 surrounded by the element substrate 610, the sealing substrate 604, and the sealant 605. The region 607 is filled with a filler. In some cases, the region 607 is filled with an inert gas (nitrogen, argon, or the like) or filled with an ultraviolet curable resin or a thermosetting resin which can be used for the sealant 605. For example, a polyvinyl chloride (PVC)-based resin, an acrylic-based resin, a polyimide-based resin, an epoxy-based resin, a silicone-based resin, a polyvinyl butyral (PVB)-based resin, or an ethylene vinyl acetate (EVA)-based resin can be used. It is preferable that the sealing substrate be provided with a recessed portion and the desiccant be provided in the recessed portion, in which case deterioration due to influence of moisture can be inhibited.

An optical element 621 is provided below the sealing substrate 604 to overlap with the light-emitting element 618. A light-blocking layer 622 is provided below the sealing substrate 604. The structures of the optical element 621 and the light-blocking layer 622 can be the same as those of the optical element and the light-blocking layer in Embodiment 3, respectively.

An epoxy-based resin or glass frit is preferably used for the sealant 605. It is preferable that such a material do not transmit moisture or oxygen as much as possible. As the sealing substrate 604, a glass substrate, a quartz substrate, or a plastic substrate formed of fiber reinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester, acrylic, or the like can be used.

In the above-described manner, the display device including any of the light-emitting elements and the optical elements which are described in Embodiments 1 to 3 can be obtained.

<Structure Example 2 of Display Device>

Next, another example of the display device is described with reference to FIGS. 13A and 13B and FIG. 14. Note that FIGS. 13A and 13B and FIG. 14 are each a cross-sectional view of a display device of one embodiment of the present invention.

In FIG. 13A, a substrate 1001, a base insulating film 1002, a gate insulating film 1003, gate electrodes 1006, 1007, and 1008, a first interlayer insulating film 1020, a second interlayer insulating film 1021, a peripheral portion 1042, a pixel portion 1040, a driver circuit portion 1041, lower electrodes 1024R, 1024G, and 1024B of light-emitting elements, a partition wall 1025, an EL layer 1028, an upper electrode 1026 of the light-emitting elements, a sealing layer 1029, a sealing substrate 1031, a sealant 1032, and the like are illustrated.

In FIG. 13A, examples of the optical elements, coloring layers (a red coloring layer 1034R, a green coloring layer 1034G, and a blue coloring layer 1034B) are provided on a transparent base material 1033. Further, a light-blocking layer 1035 may be provided. The transparent base material 1033 provided with the coloring layers and the light-blocking layer is positioned and fixed to the substrate 1001. Note that the coloring layers and the light-blocking layer are covered with an overcoat layer 1036. In the structure in FIG. 13A, red light, green light, and blue light transmit the coloring layers, and thus an image can be displayed with the use of pixels of three colors.

FIG. 13B illustrates an example in which, as examples of the optical elements, the coloring layers (the red coloring layer 1034R, the green coloring layer 1034G, and the blue coloring layer 1034B) are provided between the gate insulating film 1003 and the first interlayer insulating film 1020. As in this structure, the coloring layers may be provided between the substrate 1001 and the sealing substrate 1031.

FIG. 14 illustrates an example in which, as examples of the optical elements, the coloring layers (the red coloring layer 1034R, the green coloring layer 1034G, and the blue coloring layer 1034B) are provided between the first interlayer insulating film 1020 and the second interlayer insulating film 1021. As in this structure, the coloring layers may be provided between the substrate 1001 and the sealing substrate 1031.

The above-described display device has a structure in which light is extracted from the substrate 1001 side where the transistors are formed (a bottom-emission structure), but may have a structure in which light is extracted from the sealing substrate 1031 side (a top-emission structure).

<Structure Example 3 of Display Device>

FIGS. 15A and 15B are each an example of a cross-sectional view of a display device having a top emission structure. Note that FIGS. 15A and 15B are each a cross-sectional view illustrating the display device of one embodiment of the present invention, and the driver circuit portion 1041, the peripheral portion 1042, and the like, which are illustrated in FIGS. 13A and 13B and FIG. 14, are not illustrated therein.

In this case, as the substrate 1001, a substrate that does not transmit light can be used. The process up to the step of forming a connection electrode which connects the transistor and the anode of the light-emitting element is performed in a manner similar to that of the display device having a bottom-emission structure. Then, a third interlayer insulating film 1037 is formed to cover an electrode 1022. This insulating film may have a planarization function. The third interlayer insulating film 1037 can be formed by using a material similar to that of the second interlayer insulating film, or can be formed by using any other known materials.

The lower electrodes 1024R, 1024G, and 1024B of the light-emitting elements each function as an anode here, but may function as a cathode. Further, in the case of a display device having a top-emission structure as illustrated in FIGS. 15A and 15B, the lower electrodes 1024R, 1024G, and 1024B preferably have a function of reflecting light. The upper electrode 1026 is provided over the EL layer 1028. It is preferable that the upper electrode 1026 have a function of reflecting light and a function of transmitting light and that a microcavity structure be used between the upper electrode 1026 and the lower electrodes 1024R, 1024G, and 1024B, in which case the intensity of light having a specific wavelength is increased.

In the case of a top-emission structure as illustrated in FIG. 15A, sealing can be performed with the sealing substrate 1031 on which the coloring layers (the red coloring layer 1034R, the green coloring layer 1034G, and the blue coloring layer 1034B) are provided. The sealing substrate 1031 may be provided with the light-blocking layer 1035 which is positioned between pixels. Note that a light-transmitting substrate is favorably used as the sealing substrate 1031.

FIG. 15A illustrates the structure provided with the light-emitting elements and the coloring layers for the light-emitting elements as an example; however, the structure is not limited thereto. For example, as shown in FIG. 15B, a structure including the red coloring layer 1034R and the blue coloring layer 1034B but not including a green coloring layer may be employed to achieve full color display with the three colors of red, green, and blue. The structure as illustrated in FIG. 15A where the light-emitting elements are provided with the coloring layers is effective to suppress reflection of external light. In contrast, the structure as illustrated in FIG. 15B where the light-emitting elements are provided with the red coloring layer and the blue coloring layer and without the green coloring layer is effective to reduce power consumption because of small energy loss of light emitted from the green light-emitting element.

<Structure Example 4 of Display Device>

Although a display device including sub-pixels of three colors (red, green, and blue) is described above, the number of colors of sub-pixels may be four (red, green, blue, and yellow, or red, green, blue, and white). FIGS. 16A and 16B, FIG. 17, and FIGS. 18A and 18B illustrate structures of display devices each including the lower electrodes 1024R, 1024G, 1024B, and 1024Y. FIGS. 16A and 16B and FIG. 17 each illustrate a display device having a structure in which light is extracted from the substrate 1001 side on which transistors are formed (bottom-emission structure), and FIGS. 18A and 18B each illustrate a display device having a structure in which light is extracted from the sealing substrate 1031 side (top-emission structure).

FIG. 16A illustrates an example of a display device in which optical elements (the coloring layer 1034R, the coloring layer 1034G, the coloring layer 1034B, and a coloring layer 1034Y) are provided on the transparent base material 1033. FIG. 16B illustrates an example of a display device in which optical elements (the coloring layer 1034R, the coloring layer 1034G, and the coloring layer 1034B) are provided between the gate insulating film 1003 and the first interlayer insulating film 1020. FIG. 17 illustrates an example of a display device in which optical elements (the coloring layer 1034R, the coloring layer 1034G, the coloring layer 1034B, and the coloring layer 1034Y) are provided between the first interlayer insulating film 1020 and the second interlayer insulating film 1021.

The coloring layer 1034R transmits red light, the coloring layer 1034G transmits green light, and the coloring layer 1034B transmits blue light. The coloring layer 1034Y transmits yellow light or transmits light of a plurality of colors selected from blue, green, yellow, and red. When the coloring layer 1034Y can transmit light of a plurality of colors selected from blue, green, yellow, and red, light released from the coloring layer 1034Y may be white light. Since the light-emitting element which transmits yellow or white light has high emission efficiency, the display device including the coloring layer 1034Y can have lower power consumption.

In the top-emission display devices illustrated in FIGS. 18A and 18B, a light-emitting element including the lower electrode 1024Y preferably has a microcavity structure between the upper electrode 1026 and the lower electrodes 1024R, 1024G, 1024B, and 1024Y as in the display device illustrated in FIG. 15A. In the display device illustrated in FIG. 18A, sealing can be performed with the sealing substrate 1031 on which the coloring layers (the red coloring layer 1034R, the green coloring layer 1034G, the blue coloring layer 1034B, and the yellow coloring layer 1034Y) are provided.

Light emitted through the microcavity and the yellow coloring layer 1034Y has an emission spectrum in a yellow region. Since yellow is a color with a high luminosity factor, a light-emitting element emitting yellow light has high emission efficiency. Therefore, the display device of FIG. 18A can reduce power consumption.

FIG. 18A illustrates the structure provided with the light-emitting elements and the coloring layers for the light-emitting elements as an example; however, the structure is not limited thereto. For example, as shown in FIG. 18B, a structure including the red coloring layer 1034R, the green coloring layer 1034G, and the blue coloring layer 1034B but not including a yellow coloring layer may be employed to achieve full color display with the four colors of red, green, blue, and yellow or of red, green, blue, and white. The structure as illustrated in FIG. 18A where the light-emitting elements are provided with the coloring layers is effective to suppress reflection of external light. In contrast, the structure as illustrated in FIG. 18B where the light-emitting elements are provided with the red coloring layer, the green coloring layer, and the blue coloring layer and without the yellow coloring layer is effective to reduce power consumption because of small energy loss of light emitted from the yellow or white light-emitting element.

The structure described in this embodiment can be combined with any of the structures in this embodiment and the other embodiments.

(Embodiment 5)

In this embodiment, a display device including a light-emitting element of one embodiment of the present invention will be described with reference to FIGS. 19A and 19B, FIGS. 20A and 20B, and FIGS. 21A and 21B.

FIG. 19A is a block diagram illustrating the display device of one embodiment of the present invention, and FIG. 19B is a circuit diagram illustrating a pixel circuit of the display device of one embodiment of the present invention.

<Description of Display Device>

The display device illustrated in FIG. 19A includes a region including pixels of display elements (the region is hereinafter referred to as a pixel portion 802), a circuit portion provided outside the pixel portion 802 and including circuits for driving the pixels (the portion is hereinafter referred to as a driver circuit portion 804), circuits having a function of protecting elements (the circuits are hereinafter referred to as protection circuits 806), and a terminal portion 807. Note that the protection circuits 806 are not necessarily provided.

A part or the whole of the driver circuit portion 804 is preferably formed over a substrate over which the pixel portion 802 is formed, in which case the number of components and the number of terminals can be reduced. When a part or the whole of the driver circuit portion 804 is not formed over the substrate over which the pixel portion 802 is formed, the part or the whole of the driver circuit portion 804 can be mounted by COG or tape automated bonding (TAB).

The pixel portion 802 includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (such circuits are hereinafter referred to as pixel circuits 801). The driver circuit portion 804 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (the circuit is hereinafter referred to as a scan line driver circuit 804 a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (the circuit is hereinafter referred to as a signal line driver circuit 804 b).

The scan line driver circuit 804 a includes a shift register or the like. Through the terminal portion 807, the scan line driver circuit 804 a receives a signal for driving the shift register and outputs a signal. For example, the scan line driver circuit 804 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The scan line driver circuit 804 a has a function of controlling the potentials of wirings supplied with scan signals (such wirings are hereinafter referred to as scan lines GL_1 to GL_X). Note that a plurality of scan line driver circuits 804 a may be provided to control the scan lines GL_1 to GL_X separately. Alternatively, the scan line driver circuit 804 a has a function of supplying an initialization signal. Without being limited thereto, the scan line driver circuit 804 a can supply another signal.

The signal line driver circuit 804 b includes a shift register or the like. The signal line driver circuit 804 b receives a signal (image signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion 807. The signal line driver circuit 804 b has a function of generating a data signal to be written to the pixel circuit 801 which is based on the image signal. In addition, the signal line driver circuit 804 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the signal line driver circuit 804 b has a function of controlling the potentials of wirings supplied with data signals (such wirings are hereinafter referred to as data lines DL_1 to DL_Y). Alternatively, the signal line driver circuit 804 b has a function of supplying an initialization signal. Without being limited thereto, the signal line driver circuit 804 b can supply another signal.

The signal line driver circuit 804 b includes a plurality of analog switches or the like, for example. The signal line driver circuit 804 b can output, as the data signals, signals obtained by time-dividing the image signal by sequentially turning on the plurality of analog switches. The signal line driver circuit 804 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality of pixel circuits 801 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality of pixel circuits 801 are controlled by the scan line driver circuit 804 a. For example, to the pixel circuit 801 in the m-th row and the n-th column (m is a natural number of less than or equal to X, and n is a natural number of less than or equal to Y), a pulse signal is input from the scan line driver circuit 804 a through the scan line GL_m, and a data signal is input from the signal line driver circuit 804 b through the data line DL_n in accordance with the potential of the scan line GL_m.

The protection circuit 806 shown in FIG. 19A is connected to, for example, the scan line GL between the scan line driver circuit 804 a and the pixel circuit 801. Alternatively, the protection circuit 806 is connected to the data line DL between the signal line driver circuit 804 b and the pixel circuit 801. Alternatively, the protection circuit 806 can be connected to a wiring between the scan line driver circuit 804 a and the terminal portion 807. Alternatively, the protection circuit 806 can be connected to a wiring between the signal line driver circuit 804 b and the terminal portion 807. Note that the terminal portion 807 means a portion having terminals for inputting power, control signals, and image signals to the display device from external circuits.

The protection circuit 806 is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit.

As illustrated in FIG. 19A, the protection circuits 806 are provided for the pixel portion 802 and the driver circuit portion 804, so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of the protection circuits 806 is not limited to that, and for example, a configuration in which the protection circuits 806 are connected to the scan line driver circuit 804 a or a configuration in which the protection circuits 806 are connected to the signal line driver circuit 804 b may be employed. Alternatively, the protection circuits 806 may be configured to be connected to the terminal portion 807.

In FIG. 19A, an example in which the driver circuit portion 804 includes the scan line driver circuit 804 a and the signal line driver circuit 804 b is shown; however, the structure is not limited thereto. For example, only the scan line driver circuit 804 a may be formed and a separately prepared substrate where a signal line driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.

<Structural Example of Pixel Circuit>

Each of the plurality of pixel circuits 801 in FIG. 19A can have a structure illustrated in FIG. 19B, for example.

The pixel circuit 801 illustrated in FIG. 19B includes transistors 852 and 854, a capacitor 862, and a light-emitting element 872.

One of a source electrode and a drain electrode of the transistor 852 is electrically connected to a wiring to which a data signal is supplied (a data line DL_n). A gate electrode of the transistor 852 is electrically connected to a wiring to which a gate signal is supplied (a scan line GL_m).

The transistor 852 has a function of controlling whether to write a data signal.

One of a pair of electrodes of the capacitor 862 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 852.

The capacitor 862 functions as a storage capacitor for storing written data.

One of a source electrode and a drain electrode of the transistor 854 is electrically connected to the potential supply line VL_a. Furthermore, a gate electrode of the transistor 854 is electrically connected to the other of the source electrode and the drain electrode of the transistor 852.

One of an anode and a cathode of the light-emitting element 872 is electrically connected to a potential supply line VL_b, and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 854.

As the light-emitting element 872, any of the light-emitting elements described in Embodiments 1 to 3 can be used.

Note that a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other.

In the display device including the pixel circuits 801 in FIG. 19B, the pixel circuits 801 are sequentially selected row by row by the scan line driver circuit 804 a in FIG. 19A, for example, whereby the transistors 852 are turned on and a data signal is written.

When the transistors 852 are turned off, the pixel circuits 801 in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 854 is controlled in accordance with the potential of the written data signal. The light-emitting element 872 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image is displayed.

Alternatively, the pixel circuit can have a function of compensating variation in threshold voltages or the like of a transistor. FIGS. 20A and 20B and FIGS. 21A and 21B illustrate examples of the pixel circuit.

The pixel circuit illustrated in FIG. 20A includes six transistors (transistors 303_1 to 303_6), a capacitor 304, and a light-emitting element 305. The pixel circuit illustrated in FIG. 20A is electrically connected to wirings 301_1 to 301_5 and wirings302_1 and 302_2. Note that as the transistors 303_1 to 303_6, for example, p-channel transistors can be used.

The pixel circuit shown in FIG. 20B has a configuration in which a transistor 303_7 is added to the pixel circuit shown in FIG. 20A. The pixel circuit illustrated in FIG. 20B is electrically connected to wirings 301_6 and 301_7. The wirings 301_5 and 301_6 may be electrically connected to each other. Note that as the transistor 303_7, for example, a p-channel transistor can be used.

The pixel circuit shown in FIG. 21A includes six transistors (transistors 308_1 to 308_6), the capacitor 304, and the light-emitting element 305. The pixel circuit illustrated in FIG. 21A is electrically connected to wirings 306_1 to 306_3 and wirings 307_1 to 307_3. The wirings 306_1 and 306_3 may be electrically connected to each other. Note that as the transistors 308_1 to 308_6, for example, p-channel transistors can be used.

The pixel circuit illustrated in FIG. 21B includes two transistors (transistors 309_1 and 309_2), two capacitors (capacitors 304_1 and 304_2), and the light-emitting element 305. The pixel circuit illustrated in FIG. 21B is electrically connected to wirings 311_1 to 311_3 and wirings 312_1 and 312_2. With the configuration of the pixel circuit illustrated in FIG. 21B, the pixel circuit can be driven by a voltage inputting current driving method (also referred to as CVCC). Note that as the transistors 309_1 and 309_2, for example, p-channel transistors can be used.

A light-emitting element of one embodiment of the present invention can be used for an active matrix method in which an active element is included in a pixel of a display device or a passive matrix method in which an active element is not included in a pixel of a display device.

In the active matrix method, as an active element (a non-linear element), not only a transistor but also a variety of active elements (non-linear elements) can be used. For example, a metal insulator metal (MIM), a thin film diode (TFD), or the like can also be used. Since these elements can be formed with a smaller number of manufacturing steps, manufacturing cost can be reduced or yield can be improved. Alternatively, since the size of these elements is small, the aperture ratio can be improved, so that power consumption can be reduced and higher luminance can be achieved.

As a method other than the active matrix method, the passive matrix method in which an active element (a non-linear element) is not used can also be used. Since an active element (a non-linear element) is not used, the number of manufacturing steps is small, so that manufacturing cost can be reduced or yield can be improved. Alternatively, since an active element (a non-linear element) is not used, the aperture ratio can be improved, so that power consumption can be reduced or higher luminance can be achieved, for example.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

(Embodiment 6)

In this embodiment, a display device including a light-emitting element of one embodiment of the present invention and an electronic device in which the display device is provided with an input device will be described with reference to FIGS. 22A and 22B, FIGS. 23A to 23C, FIGS. 24A and 24B, FIGS. 25A and 25B, and FIG. 26.

<Description 1 of Touch Panel>

In this embodiment, a touch panel 2000 including a display device and an input device will be described as an example of an electronic device. In addition, an example in which a touch sensor is used as an input device will be described.

FIGS. 22A and 22B are perspective views of the touch panel 2000. Note that FIGS. 22A and 22B illustrate only main components of the touch panel 2000 for simplicity.

The touch panel 2000 includes a display device 2501 and a touch sensor 2595 (see FIG. 22B). The touch panel 2000 also includes a substrate 2510, a substrate 2570, and a substrate 2590. The substrate 2510, the substrate 2570, and the substrate 2590 each have flexibility. Note that one or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display device 2501 includes a plurality of pixels over the substrate 2510 and a plurality of wirings 2511 through which signals are supplied to the pixels. The plurality of wirings 2511 are led to a peripheral portion of the substrate 2510, and parts of the plurality of wirings 2511 form a terminal 2519. The terminal 2519 is electrically connected to an FPC 2509(1). The plurality of wirings 2511 can supply signals from a signal line driver circuit 2503 s(1) to the plurality of pixels.

The substrate 2590 includes the touch sensor 2595 and a plurality of wirings 2598 electrically connected to the touch sensor 2595. The plurality of wirings 2598 are led to a peripheral portion of the substrate 2590, and parts of the plurality of wirings 2598 form a terminal. The terminal is electrically connected to an FPC 2509(2). Note that in FIG. 22B, electrodes, wirings, and the like of the touch sensor 2595 provided on the back side of the substrate 2590 (the side facing the substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used. Examples of the capacitive touch sensor are a surface capacitive touch sensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self capacitive touch sensor and a mutual capacitive touch sensor, which differ mainly in the driving method. The use of a mutual capacitive type is preferable because multiple points can be sensed simultaneously.

Note that the touch sensor 2595 illustrated in FIG. 22B is an example of using a projected capacitive touch sensor.

Note that a variety of sensors that can sense approach or contact of a sensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 and electrodes 2592. The electrodes 2591 are electrically connected to any of the plurality of wirings 2598, and the electrodes 2592 are electrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadrangles arranged in one direction with one corner of a quadrangle connected to one corner of another quadrangle as illustrated in FIGS. 22A and 22B.

The electrodes 2591 each have a quadrangular shape and are arranged in a direction intersecting with the direction in which the electrodes 2592 extend.

A wiring 2594 electrically connects two electrodes 2591 between which the electrode 2592 is positioned. The intersecting area of the electrode 2592 and the wiring 2594 is preferably as small as possible. Such a structure allows a reduction in the area of a region where the electrodes are not provided, reducing variation in transmittance. As a result, variation in luminance of light passing through the touch sensor 2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 are not limited thereto and can be any of a variety of shapes. For example, a structure may be employed in which the plurality of electrodes 2591 are arranged so that gaps between the electrodes 2591 are reduced as much as possible, and the electrodes 2592 are spaced apart from the electrodes 2591 with an insulating layer interposed therebetween to have regions not overlapping with the electrodes 2591. In this case, it is preferable to provide, between two adjacent electrodes 2592, a dummy electrode electrically insulated from these electrodes because the area of regions having different transmittances can be reduced.

<Description of Display Device>

Next, the display device 2501 will be described in detail with reference to FIG. 23A. FIG. 23A corresponds to a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 22B.

The display device 2501 includes a plurality of pixels arranged in a matrix. Each of the pixels includes a display element and a pixel circuit for driving the display element.

In the following description, an example of using a light-emitting element that emits white light as a display element will be described; however, the display element is not limited to such an element. For example, light-emitting elements that emit light of different colors may be included so that the light of different colors can be emitted from adjacent pixels.

For the substrate 2510 and the substrate 2570, for example, a flexible material with a vapor permeability of lower than or equal to 1×10⁻⁵ g·m⁻²·day⁻¹, preferably lower than or equal to 1×10⁻⁶ g·m⁻²·day⁻¹ can be favorably used. Alternatively, materials whose thermal expansion coefficients are substantially equal to each other are preferably used for the substrate 2510 and the substrate 2570. For example, the coefficients of linear expansion of the materials are preferably lower than or equal to 1×10⁻³/K, further preferably lower than or equal to 5×10⁻⁵/K, and still further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulating layer 2510 a for preventing impurity diffusion into the light-emitting element, a flexible substrate 2510 b, and an adhesive layer 2510 c for attaching the insulating layer 2510 a and the flexible substrate 2510 b to each other. The substrate 2570 is a stacked body including an insulating layer 2570 a for preventing impurity diffusion into the light-emitting element, a flexible substrate 2570 b, and an adhesive layer 2570 c for attaching the insulating layer 2570 a and the flexible substrate 2570 b to each other.

For the adhesive layer 2510 c and the adhesive layer 2570 c, for example, polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, or acrylic, urethane, or epoxy can be used. Alternatively, a material that includes a resin having a siloxane bond can be used.

A sealing layer 2560 is provided between the substrate 2510 and the substrate 2570. The sealing layer 2560 preferably has a refractive index higher than that of air. In the case where light is extracted to the sealing layer 2560 side as illustrated in FIG. 23A, the sealing layer 2560 can also serve as an optical adhesive layer.

A sealant may be formed in the peripheral portion of the sealing layer 2560. With the use of the sealant, a light-emitting element 2550R can be provided in a region surrounded by the substrate 2510, the substrate 2570, the sealing layer 2560, and the sealant. Note that an inert gas (such as nitrogen and argon) may be used instead of the sealing layer 2560. A drying agent may be provided in the inert gas so as to adsorb moisture or the like. An epoxy-based resin or a glass frit is preferably used as the sealant. As a material used for the sealant, a material which is impermeable to moisture and oxygen is preferably used.

The display device 2501 includes a pixel 2502R. The pixel 2502R includes a light-emitting module 2580R.

The pixel 2502R includes the light-emitting element 2550R and a transistor 2502 t that can supply electric power to the light-emitting element 2550R. Note that the transistor 2502 t functions as part of the pixel circuit. The light-emitting module 2580R includes the light-emitting element 2550R and a coloring layer 2567R.

The light-emitting element 2550R includes a lower electrode, an upper electrode, and an EL layer between the lower electrode and the upper electrode. As the light-emitting element 2550R, any of the light-emitting elements described in Embodiments 1 to 3 can be used.

A microcavity structure may be employed between the lower electrode and the upper electrode so as to increase the intensity of light having a specific wavelength.

In the case where the sealing layer 2560 is provided on the light extraction side, the sealing layer 2560 is in contact with the light-emitting element 2550R and the coloring layer 2567R.

The coloring layer 2567R is positioned in a region overlapping with the light-emitting element 2550R. Accordingly, part of light emitted from the light-emitting element 2550R passes through the coloring layer 2567R and is emitted to the outside of the light-emitting module 2580R as indicated by an arrow in FIG. 23A.

The display device 2501 includes a light-blocking layer 2567BM on the light extraction side. The light-blocking layer 2567BM is provided so as to surround the coloring layer 2567R.

The coloring layer 2567R is a coloring layer having a function of transmitting light in a particular wavelength range. For example, a color filter for transmitting light in a red wavelength range, a color filter for transmitting light in a green wavelength range, a color filter for transmitting light in a blue wavelength range, a color filter for transmitting light in a yellow wavelength range, or the like can be used. Each color filter can be formed with any of various materials by a printing method, an inkjet method, an etching method using a photolithography technique, or the like.

An insulating layer 2521 is provided in the display device 2501. The insulating layer 2521 covers the transistor 2502 t. Note that the insulating layer 2521 has a function of covering unevenness caused by the pixel circuit. The insulating layer 2521 may have a function of suppressing impurity diffusion. This can prevent the reliability of the transistor 2502 t or the like from being lowered by impurity diffusion.

The light-emitting element 2550R is formed over the insulating layer 2521. A partition 2528 is provided so as to overlap with an end portion of the lower electrode of the light-emitting element 2550R. Note that a spacer for controlling the distance between the substrate 2510 and the substrate 2570 may be formed over the partition 2528.

A scan line driver circuit 2503 g(1) includes a transistor 2503 t and a capacitor 2503 c. Note that the driver circuit can be formed in the same process and over the same substrate as those of the pixel circuits.

The wirings 2511 through which signals can be supplied are provided over the substrate 2510. The terminal 2519 is provided over the wirings 2511. The FPC 2509(1) is electrically connected to the terminal 2519. The FPC 2509(1) has a function of supplying a video signal, a clock signal, a start signal, a reset signal, or the like. Note that the FPC 2509(1) may be provided with a PWB.

In the display device 2501, transistors with any of a variety of structures can be used. FIG. 23A illustrates an example of using bottom-gate transistors; however, the present invention is not limited to this example, and top-gate transistors may be used in the display device 2501 as illustrated in FIG. 23B.

In addition, there is no particular limitation on the polarity of the transistor 2502 t and the transistor 2503 t. For these transistors, n-channel and p-channel transistors may be used, or either n-channel transistors or p-channel transistors may be used, for example. Furthermore, there is no particular limitation on the crystallinity of a semiconductor film used for the transistors 2502 t and 2503 t. For example, an amorphous semiconductor film or a crystalline semiconductor film may be used. Examples of semiconductor materials include Group 14 semiconductors (e.g., a semiconductor including silicon), compound semiconductors (including oxide semiconductors), organic semiconductors, and the like. An oxide semiconductor that has an energy gap of 2 eV or more, preferably 2.5 eV or more, further preferably 3 eV or more is preferably used for one of the transistors 2502 t and 2503 t or both, so that the off-state current of the transistors can be reduced. Examples of the oxide semiconductors include an In—Ga oxide, an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, Sn, Hf, or Nd), and the like.

<Description of Touch Sensor>

Next, the touch sensor 2595 will be described in detail with reference to FIG. 23C. FIG. 23C corresponds to a cross-sectional view taken along dashed-dotted line X3-X4 in FIG. 22B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes 2592 provided in a staggered arrangement on the substrate 2590, an insulating layer 2593 covering the electrodes 2591 and the electrodes 2592, and the wiring 2594 that electrically connects the adjacent electrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using a light-transmitting conductive material. As a light-transmitting conductive material, a conductive oxide such as indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added can be used. Note that a film including graphene may be used as well. The film including graphene can be formed, for example, by reducing a film containing graphene oxide. As a reducing method, a method with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, for example, depositing a light-transmitting conductive material on the substrate 2590 by a sputtering method and then removing an unnecessary portion by any of various pattern forming techniques such as photolithography.

Examples of a material for the insulating layer 2593 are a resin such as an acrylic resin or an epoxy resin, a resin having a siloxane bond, and an inorganic insulating material such as silicon oxide, silicon oxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer 2593, and the wiring 2594 electrically connects the adjacent electrodes 2591. A light-transmitting conductive material can be favorably used as the wiring 2594 because the aperture ratio of the touch panel can be increased. Moreover, a material with higher conductivity than the conductivities of the electrodes 2591 and 2592 can be favorably used for the wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality of electrodes 2592 are provided in the form of stripes. The wiring 2594 intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 provided therebetween. The wiring 2594 electrically connects the adjacent electrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arranged in the direction orthogonal to one electrode 2592 and may be arranged to intersect with one electrode 2592 at an angle of more than 0 degrees and less than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591 and 2592. Part of the wiring 2598 functions as a terminal. For the wiring 2598, a metal material such as aluminum, gold, platinum, silver, nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy material containing any of these metal materials can be used.

Note that an insulating layer that covers the insulating layer 2593 and the wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC 2509(2).

As the connection layer 2599, any of various anisotropic conductive films (ACF), anisotropic conductive pastes (ACP), and the like can be used.

<Description 2 of Touch Panel>

Next, the touch panel 2000 will be described in detail with reference to FIG. 24A. FIG. 24A corresponds to a cross-sectional view taken along dashed-dotted line X5-X6 in FIG. 22A.

In the touch panel 2000 illustrated in FIG. 24A, the display device 2501 described with reference to FIG. 23A and the touch sensor 2595 described with reference to FIG. 23C are attached to each other.

The touch panel 2000 illustrated in FIG. 24A includes an adhesive layer 2597 and an anti-reflective layer 2567 p in addition to the components described with reference to FIGS. 23A and 23C.

The adhesive layer 2597 is provided in contact with the wiring 2594. Note that the adhesive layer 2597 attaches the substrate 2590 to the substrate 2570 so that the touch sensor 2595 overlaps with the display device 2501. The adhesive layer 2597 preferably has a light-transmitting property. A heat curable resin or an ultraviolet curable resin can be used for the adhesive layer 2597. For example, an acrylic resin, a urethane-based resin, an epoxy-based resin, or a siloxane-based resin can be used.

The anti-reflective layer 2567 p is positioned in a region overlapping with pixels. As the anti-reflective layer 2567 p, a circularly polarizing plate can be used, for example.

Next, a touch panel having a structure different from that illustrated in FIG. 24A will be described with reference to FIG. 24B.

FIG. 24B is a cross-sectional view of a touch panel 2001. The touch panel 2001 illustrated in FIG. 24B differs from the touch panel 2000 illustrated in FIG. 24A in the position of the touch sensor 2595 relative to the display device 2501. Different parts are described in detail below, and the above description of the touch panel 2000 is referred to for the other similar parts.

The coloring layer 2567R is positioned in a region overlapping with the light-emitting element 2550R. The light-emitting element 2550R illustrated in FIG. 24B emits light to the side where the transistor 2502 t is provided. Accordingly, part of light emitted from the light-emitting element 2550R passes through the coloring layer 2567R and is emitted to the outside of the light-emitting module 2580R as indicated by an arrow in FIG. 24B.

The touch sensor 2595 is provided on the substrate 2510 side of the display device 2501.

The adhesive layer 2597 is provided between the substrate 2510 and the substrate 2590 and attaches the touch sensor 2595 to the display device 2501.

As illustrated in FIG. 24A or 24B, light may be emitted from the light-emitting element to one or both of upper and lower sides of the substrate 2510.

<Description of Method for Driving Touch Panel>

Next, an example of a method for driving a touch panel will be described with reference to FIGS. 25A and 25B.

FIG. 25A is a block diagram illustrating the structure of a mutual capacitive touch sensor. FIG. 25A illustrates a pulse voltage output circuit 2601 and a current sensing circuit 2602. Note that in FIG. 25A, six wirings X1 to X6 represent the electrodes 2621 to which a pulse voltage is applied, and six wirings Y1 to Y6 represent the electrodes 2622 that detect changes in current. FIG. 25A also illustrates capacitors 2603 that are each formed in a region where the electrodes 2621 and 2622 overlap with each other. Note that functional replacement between the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentially applying a pulse voltage to the wirings X1 to X6. By application of a pulse voltage to the wirings X1 to X6, an electric field is generated between the electrodes 2621 and 2622 of the capacitor 2603. When the electric field between the electrodes is shielded, for example, a change occurs in the capacitor 2603 (mutual capacitance). The approach or contact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes in current flowing through the wirings Y1 to Y6 that are caused by the change in mutual capacitance in the capacitor 2603. No change in current value is detected in the wirings Y1 to Y6 when there is no approach or contact of a sensing target, whereas a decrease in current value is detected when mutual capacitance is decreased owing to the approach or contact of a sensing target. Note that an integrator circuit or the like is used for sensing of current values.

FIG. 25B is a timing chart showing input and output waveforms in the mutual capacitive touch sensor illustrated in FIG. 25A. In FIG. 25B, sensing of a sensing target is performed in all the rows and columns in one frame period. FIG. 25B shows a period when a sensing target is not sensed (not touched) and a period when a sensing target is sensed (touched). In FIG. 25B, sensed current values of the wirings Y1 to Y6 are shown as the waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and the waveforms of the wirings Y1 to Y6 change in accordance with the pulse voltage. When there is no approach or contact of a sensing target, the waveforms of the wirings Y1 to Y6 change in accordance with changes in the voltages of the wirings X1 to X6. The current value is decreased at the point of approach or contact of a sensing target and accordingly the waveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approach or contact of a sensing target can be sensed.

<Description of Sensor Circuit>

Although FIG. 25A illustrates a passive matrix type touch sensor in which only the capacitor 2603 is provided at the intersection of wirings as a touch sensor, an active matrix type touch sensor including a transistor and a capacitor may be used. FIG. 26 illustrates an example of a sensor circuit included in an active matrix type touch sensor.

The sensor circuit in FIG. 26 includes the capacitor 2603 and transistors 2611, 2612, and 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES is applied to one of a source and a drain of the transistor 2613, and one electrode of the capacitor 2603 and a gate of the transistor 2611 are electrically connected to the other of the source and the drain of the transistor 2613. One of a source and a drain of the transistor 2611 is electrically connected to one of a source and a drain of the transistor 2612, and a voltage VSS is applied to the other of the source and the drain of the transistor 2611. A signal G1 is input to a gate of the transistor 2612, and a wiring ML is electrically connected to the other of the source and the drain of the transistor 2612. The voltage VSS is applied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 26 will be described. First, a potential for turning on the transistor 2613 is supplied as the signal G2, and a potential with respect to the voltage VRES is thus applied to the node n connected to the gate of the transistor 2611. Then, a potential for turning off the transistor 2613 is applied as the signal G2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to the approach or contact of a sensing target such as a finger, and accordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 is supplied as the signal G1. A current flowing through the transistor 2611, that is, a current flowing through the wiring ML is changed in accordance with the potential of the node n. By sensing this current, the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, an oxide semiconductor layer is preferably used as a semiconductor layer in which a channel region is formed. In particular, such a transistor is preferably used as the transistor 2613 so that the potential of the node n can be held for a long time and the frequency of operation of resupplying VRES to the node n (refresh operation) can be reduced.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

(Embodiment 7)

In this embodiment, a display module and electronic devices including a light-emitting element of one embodiment of the present invention will be described with reference to FIG. 27 and FIGS. 28A to 28G.

<Description of Display Module>

In a display module 8000 in FIG. 27, a touch sensor 8004 connected to an FPC 8003, a display device 8006 connected to an FPC 8005, a frame 8009, a printed board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002.

The light-emitting element of one embodiment of the present invention can be used for the display device 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch sensor 8004 and the display device 8006.

The touch sensor 8004 can be a resistive touch sensor or a capacitive touch sensor and may be formed to overlap with the display device 8006. A counter substrate (sealing substrate) of the display device 8006 can have a touch sensor function. A photosensor may be provided in each pixel of the display device 8006 so that an optical touch sensor is obtained.

The frame 8009 protects the display device 8006 and also serves as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 8010. The frame 8009 may serve as a radiator plate.

The printed board 8010 has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the battery 8011 provided separately may be used. The battery 8011 can be omitted in the case of using a commercial power source.

The display module 8000 can be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

<Description of Electronic Device>

FIGS. 28A to 28G illustrate electronic devices. These electronic devices can include a housing 9000, a display portion 9001, a speaker 9003, operation keys 9005 (including a power switch or an operation switch), a connection terminal 9006, a sensor 9007 (a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 9008, and the like.

The electronic devices illustrated in FIGS. 28A to 28G can have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch sensor function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like. Note that functions that can be provided for the electronic devices illustrated in FIGS. 28A to 28G are not limited to those described above, and the electronic devices can have a variety of functions. Although not illustrated in FIGS. 28A to 28G, the electronic devices may include a plurality of display portions. The electronic devices may have a camera or the like and a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like.

The electronic devices illustrated in FIGS. 28A to 28G will be described in detail below.

FIG. 28A is a perspective view of a portable information terminal 9100. The display portion 9001 of the portable information terminal 9100 is flexible. Therefore, the display portion 9001 can be incorporated along a bent surface of a bent housing 9000. In addition, the display portion 9001 includes a touch sensor, and operation can be performed by touching the screen with a finger, a stylus, or the like. For example, when an icon displayed on the display portion 9001 is touched, an application can be started.

FIG. 28B is a perspective view of a portable information terminal 9101. The portable information terminal 9101 functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal can be used as a smartphone. Note that the speaker 9003, the connection terminal 9006, the sensor 9007, and the like, which are not shown in FIG. 28B, can be positioned in the portable information terminal 9101 as in the portable information terminal 9100 shown in FIG. 28A. The portable information terminal 9101 can display characters and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons, or simply, icons) can be displayed on one surface of the display portion 9001. Furthermore, information 9051 indicated by dashed rectangles can be displayed on another surface of the display portion 9001. Examples of the information 9051 include display indicating reception of an incoming email, social networking service (SNS) message, call, and the like; the title and sender of an email and SNS message; the date; the time; remaining battery; and the reception strength of an antenna. Instead of the information 9051, the operation buttons 9050 or the like may be displayed on the position where the information 9051 is displayed.

FIG. 28C is a perspective view of a portable information terminal 9102. The portable information terminal 9102 has a function of displaying information on three or more surfaces of the display portion 9001. Here, information 9052, information 9053, and information 9054 are displayed on different surfaces. For example, a user of the portable information terminal 9102 can see the display (here, the information 9053) with the portable information terminal 9102 put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal 9102. Thus, the user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call.

FIG. 28D is a perspective view of a watch-type portable information terminal 9200. The portable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, viewing and editing texts, music reproduction, Internet communication, and computer games. The display surface of the display portion 9001 is bent, and images can be displayed on the bent display surface. The portable information terminal 9200 can employ near field communication that is a communication method based on an existing communication standard. In that case, for example, mutual communication between the portable information terminal 9200 and a headset capable of wireless communication can be performed, and thus hands-free calling is possible. The portable information terminal 9200 includes the connection terminal 9006, and data can be directly transmitted to and received from another information terminal via a connector. Power charging through the connection terminal 9006 is possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006.

FIGS. 28E, 28F, and 28G are perspective views of a foldable portable information terminal 9201. FIG. 28E is a perspective view illustrating the portable information terminal 9201 that is opened. FIG. 28F is a perspective view illustrating the portable information terminal 9201 that is being opened or being folded. FIG. 28G is a perspective view illustrating the portable information terminal 9201 that is folded. The portable information terminal 9201 is highly portable when folded. When the portable information terminal 9201 is opened, a seamless large display region is highly browsable. The display portion 9001 of the portable information terminal 9201 is supported by three housings 9000 joined together by hinges 9055. By folding the portable information terminal 9201 at a connection portion between two housings 9000 with the hinges 9055, the portable information terminal 9201 can be reversibly changed in shape from an opened state to a folded state. For example, the portable information terminal 9201 can be bent with a radius of curvature of greater than or equal to 1 mm and less than or equal to 150 mm.

The electronic devices described in this embodiment each include the display portion for displaying some sort of data. Note that the light-emitting element of one embodiment of the present invention can also be used for an electronic device which does not have a display portion. The structure in which the display portion of the electronic device described in this embodiment is flexible and display can be performed on the bent display surface or the structure in which the display portion of the electronic device is foldable is described as an example; however, the structure is not limited thereto and a structure in which the display portion of the electronic device is not flexible and display is performed on a plane portion may be employed.

The structure described in this embodiment can be used in appropriate combination with the structure described in any of the other embodiments.

(Embodiment 8)

In this embodiment, a light-emitting device including the light-emitting element of one embodiment of the present invention will be described with reference to FIGS. 29A to 29C and FIGS. 30A to 30D.

FIG. 29A is a perspective view of a light-emitting device 3000 shown in this embodiment, and FIG. 29B is a cross-sectional view along dashed-dotted line E-F in FIG. 29A. Note that in FIG. 29A, some components are illustrated by broken lines in order to avoid complexity of the drawing.

The light-emitting device 3000 illustrated in FIGS. 29A and 29B includes a substrate 3001, a light-emitting element 3005 over the substrate 3001, a first sealing region 3007 provided around the light-emitting element 3005, and a second sealing region 3009 provided around the first sealing region 3007.

Light is emitted from the light-emitting element 3005 through one or both of the substrate 3001 and a substrate 3003. In FIGS. 29A and 29B, a structure in which light is emitted from the light-emitting element 3005 to the lower side (the substrate 3001 side) is illustrated.

As illustrated in FIGS. 29A and 29B, the light-emitting device 3000 has a double sealing structure in which the light-emitting element 3005 is surrounded by the first sealing region 3007 and the second sealing region 3009. With the double sealing structure, entry of impurities (e.g., water, oxygen, and the like) from the outside into the light-emitting element 3005 can be favorably suppressed. Note that it is not necessary to provide both the first sealing region 3007 and the second sealing region 3009. For example, only the first sealing region 3007 may be provided.

Note that in FIG. 29B, the first sealing region 3007 and the second sealing region 3009 are each provided in contact with the substrate 3001 and the substrate 3003. However, without limitation to such a structure, for example, one or both of the first sealing region 3007 and the second sealing region 3009 may be provided in contact with an insulating film or a conductive film provided on the substrate 3001. Alternatively, one or both of the first sealing region 3007 and the second sealing region 3009 may be provided in contact with an insulating film or a conductive film provided on the substrate 3003.

The substrate 3001 and the substrate 3003 can have structures similar to those of the substrate 200 and the substrate 220 described in Embodiment 3, respectively. The light-emitting element 3005 can have a structure similar to that of any of the light-emitting elements described in Embodiments 1 to 3.

For the first sealing region 3007, a material containing glass (e.g., a glass frit, a glass ribbon, and the like) can be used. For the second sealing region 3009, a material containing a resin can be used. With the use of the material containing glass for the first sealing region 3007, productivity and a sealing property can be improved. Moreover, with the use of the material containing a resin for the second sealing region 3009, impact resistance and heat resistance can be improved. However, the materials used for the first sealing region 3007 and the second sealing region 3009 are not limited to such, and the first sealing region 3007 may be formed using the material containing a resin and the second sealing region 3009 may be formed using the material containing glass.

The glass frit may contain, for example, magnesium oxide, calcium oxide, strontium oxide, barium oxide, cesium oxide, sodium oxide, potassium oxide, boron oxide, vanadium oxide, zinc oxide, tellurium oxide, aluminum oxide, silicon dioxide, lead oxide, tin oxide, phosphorus oxide, ruthenium oxide, rhodium oxide, iron oxide, copper oxide, manganese dioxide, molybdenum oxide, niobium oxide, titanium oxide, tungsten oxide, bismuth oxide, zirconium oxide, lithium oxide, antimony oxide, lead borate glass, tin phosphate glass, vanadate glass, or borosilicate glass. The glass frit preferably contains at least one kind of transition metal to absorb infrared light.

As the above glass frits, for example, a frit paste is applied to a substrate and is subjected to heat treatment, laser light irradiation, or the like. The frit paste contains the glass frit and a resin (also referred to as a binder) diluted by an organic solvent. Note that an absorber which absorbs light having the wavelength of laser light may be added to the glass frit. For example, an Nd:YAG laser or a semiconductor laser is preferably used as the laser. The shape of laser light may be circular or quadrangular.

As the above material containing a resin, for example, materials that include polyester, polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate, an acrylic resin, urethane, an epoxy resin, or a resin having a siloxane bond can be used.

Note that in the case where the material containing glass is used for one or both of the first sealing region 3007 and the second sealing region 3009, the material containing glass preferably has a thermal expansion coefficient close to that of the substrate 3001. With the above structure, generation of a crack in the material containing glass or the substrate 3001 due to thermal stress can be suppressed.

For example, the following advantageous effect can be obtained in the case where the material containing glass is used for the first sealing region 3007 and the material containing a resin is used for the second sealing region 3009.

The second sealing region 3009 is provided closer to an outer portion of the light-emitting device 3000 than the first sealing region 3007 is. In the light-emitting device 3000, distortion due to external force or the like increases toward the outer portion. Thus, the outer portion of the light-emitting device 3000 where a larger amount of distortion is generated, that is, the second sealing region 3009 is sealed using the material containing a resin and the first sealing region 3007 provided on an inner side of the second sealing region 3009 is sealed using the material containing glass, whereby the light-emitting device 3000 is less likely to be damaged even when distortion due to external force or the like is generated.

Furthermore, as illustrated in FIG. 29B, a first region 3011 corresponds to the region surrounded by the substrate 3001, the substrate 3003, the first sealing region 3007, and the second sealing region 3009. A second region 3013 corresponds to the region surrounded by the substrate 3001, the substrate 3003, the light-emitting element 3005, and the first sealing region 3007.

The first region 3011 and the second region 3013 are preferably filled with, for example, an inert gas such as a rare gas or a nitrogen gas. Note that for the first region 3011 and the second region 3013, a reduced pressure state is preferred to an atmospheric pressure state.

FIG. 29C illustrates a modification example of the structure in FIG. 29B. FIG. 29C is a cross-sectional view illustrating the modification example of the light-emitting device 3000.

FIG. 29C illustrates a structure in which a desiccant 3018 is provided in a recessed portion provided in part of the substrate 3003. The other components are the same as those of the structure illustrated in FIG. 29B.

As the desiccant 3018, a substance which adsorbs moisture and the like by chemical adsorption or a substance which adsorbs moisture and the like by physical adsorption can be used. Examples of the substance that can be used as the desiccant 3018 include alkali metal oxides, alkaline earth metal oxide (e.g., calcium oxide, barium oxide, and the like), sulfate, metal halides, perchlorate, zeolite, silica gel, and the like.

Next, modification examples of the light-emitting device 3000 which is illustrated in FIG. 29B are described with reference to FIGS. 30A to 30D. Note that FIGS. 30A to 30D are cross-sectional views illustrating the modification examples of the light-emitting device 3000 illustrated in FIG. 29B.

In each of the light-emitting devices illustrated in FIGS. 30A to 30D, the second sealing region 3009 is not provided but only the first sealing region 3007 is provided. Moreover, in each of the light-emitting devices illustrated in FIGS. 30A to 30D, a region 3014 is provided instead of the second region 3013 illustrated in FIG. 29B.

For the region 3014, for example, materials that include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, an acrylic resin, an epoxy resin, urethane, an epoxy resin, or a resin having a siloxane bond can be used.

When the above-described material is used for the region 3014, what is called a solid-sealing light-emitting device can be obtained.

In the light-emitting device illustrated in FIG. 30B, a substrate 3015 is provided on the substrate 3001 side of the light-emitting device illustrated in FIG. 30A.

The substrate 3015 has unevenness as illustrated in FIG. 30B. With a structure in which the substrate 3015 having unevenness is provided on the side through which light emitted from the light-emitting element 3005 is extracted, the efficiency of extraction of light from the light-emitting element 3005 can be improved. Note that instead of the structure having unevenness and illustrated in FIG. 30B, a substrate having a function as a diffusion plate may be provided.

In the light-emitting device illustrated in FIG. 30C, light is extracted through the substrate 3003 side, unlike in the light-emitting device illustrated in FIG. 30A, in which light is extracted through the substrate 3001 side.

The light-emitting device illustrated in FIG. 30C includes the substrate 3015 on the substrate 3003 side. The other components are the same as those of the light-emitting device illustrated in FIG. 30B.

In the light-emitting device illustrated in FIG. 30D, the substrate 3003 and the substrate 3015 included in the light-emitting device illustrated in FIG. 30C are not provided but a substrate 3016 is provided.

The substrate 3016 includes first unevenness positioned closer to the light-emitting element 3005 and second unevenness positioned farther from the light-emitting element 3005. With the structure illustrated in FIG. 30D, the efficiency of extraction of light from the light-emitting element 3005 can be further improved.

Thus, the use of the structure described in this embodiment can provide a light-emitting device in which deterioration of a light-emitting element due to impurities such as moisture and oxygen is suppressed. Alternatively, with the structure described in this embodiment, a light-emitting device having high light extraction efficiency can be obtained.

Note that the structure described in this embodiment can be combined with the structure described in any of the other embodiments as appropriate.

(Embodiment 9)

In this embodiment, examples in which the light-emitting element of one embodiment of the present invention is used for various lighting devices and electronic devices will be described with reference to FIGS. 31A to 31C and FIG. 32.

An electronic device or a lighting device that has a light-emitting region with a curved surface can be obtained with the use of the light-emitting element of one embodiment of the present invention which is manufactured over a substrate having flexibility.

Furthermore, a light-emitting device to which one embodiment of the present invention is applied can also be used for lighting for motor vehicles, examples of which are lighting for a dashboard, a windshield, a ceiling, and the like.

FIG. 31A is a perspective view illustrating one surface of a multifunction terminal 3500, and FIG. 31B is a perspective view illustrating the other surface of the multifunction terminal 3500. In a housing 3502 of the multifunction terminal 3500, a display portion 3504, a camera 3506, lighting 3508, and the like are incorporated. The light-emitting device of one embodiment of the present invention can be used for the lighting 3508.

The lighting 3508 that includes the light-emitting device of one embodiment of the present invention functions as a planar light source. Thus, unlike a point light source typified by an LED, the lighting 3508 can provide light emission with low directivity. When the lighting 3508 and the camera 3506 are used in combination, for example, imaging can be performed by the camera 3506 with the lighting 3508 lighting or flashing. Because the lighting 3508 functions as a planar light source, a photograph as if taken under natural light can be taken.

Note that the multifunction terminal 3500 illustrated in FIGS. 31A and 31B can have a variety of functions as in the electronic devices illustrated in FIGS. 28A to 28G.

The housing 3502 can include a speaker, a sensor (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared rays), a microphone, and the like. When a detection device including a sensor for detecting inclination, such as a gyroscope or an acceleration sensor, is provided inside the multifunction terminal 3500, display on the screen of the display portion 3504 can be automatically switched by determining the orientation of the multifunction terminal 3500 (whether the multifunction terminal is placed horizontally or vertically for a landscape mode or a portrait mode).

The display portion 3504 may function as an image sensor. For example, an image of a palm print, a fingerprint, or the like is taken when the display portion 3504 is touched with the palm or the finger, whereby personal authentication can be performed. Furthermore, by providing a backlight or a sensing light source which emits near-infrared light in the display portion 3504, an image of a finger vein, a palm vein, or the like can be taken. Note that the light-emitting device of one embodiment of the present invention may be used for the display portion 3504.

FIG. 31C is a perspective view of a security light 3600. The security light 3600 includes lighting 3608 on the outside of the housing 3602, and a speaker 3610 and the like are incorporated in the housing 3602. The light-emitting device of one embodiment of the present invention can be used for the lighting 3608.

The security light 3600 emits light when the lighting 3608 is gripped or held, for example. An electronic circuit that can control the manner of light emission from the security light 3600 may be provided in the housing 3602. The electronic circuit may be a circuit that enables light emission once or intermittently plural times or may be a circuit that can adjust the amount of emitted light by controlling the current value for light emission. A circuit with which a loud audible alarm is output from the speaker 3610 at the same time as light emission from the lighting 3608 may be incorporated.

The security light 3600 can emit light in various directions; therefore, it is possible to intimidate a thug or the like with light, or light and sound. Moreover, the security light 3600 may include a camera such as a digital still camera to have a photography function.

FIG. 32 illustrates an example in which the light-emitting element is used for an indoor lighting device 8501. Since the light-emitting element can have a larger area, a lighting device having a large area can also be formed. In addition, a lighting device 8502 in which a light-emitting region has a curved surface can also be formed with the use of a housing with a curved surface. A light-emitting element described in this embodiment is in the form of a thin film, which allows the housing to be designed more freely. Therefore, the lighting device can be elaborately designed in a variety of ways. Furthermore, a wall of the room may be provided with a large-sized lighting device 8503. Touch sensors may be provided in the lighting devices 8501, 8502, and 8503 to control the power on/off of the lighting devices.

Moreover, when the light-emitting element is used on the surface side of a table, a lighting device 8504 which has a function as a table can be obtained. When the light-emitting element is used as part of other furniture, a lighting device which has a function as the furniture can be obtained.

As described above, lighting devices and electronic devices can be obtained by application of the light-emitting device of one embodiment of the present invention. Note that the light-emitting device can be used for lighting devices and electronic devices in a variety of fields without being limited to the lighting devices and the electronic devices described in this embodiment.

Note that the structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

EXAMPLE 1

In Example 1, examples of fabricating Light-emitting element 1 and Light-emitting element 2 (Element 1 and Element 2) of embodiments of the present invention will be described. In addition, examples of fabricating Comparative light-emitting element 1 and Comparative light-emitting element 2 (Comparative Element 1 and Comparative Element 2) will be described. Schematic cross-sectional views of light-emitting elements fabricated in Example 1 are similar to that of the light-emitting element 250 in FIG. 1A. Table 2 shows the detailed structures of the elements. In addition, structures and abbreviations of compounds used in Example 1 are given below. Note that Embodiment 1 can be referred to for other compounds.

TABLE 2 Film Reference thickness Layer numeral (nm) Material Weight ratio Light-emitting Electrode 102 200 Al — element 1 Electron-injection layer 119 1 LiF — Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:PCzPCA1:TBRb 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Light-emitting Electrode 102 200 Al — element 2 Electron-injection layer 119 1 LiF — Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4mCzBPBfpm — Light-emitting layer 130 40 4mCzBPBfpm:PCzPCA1:TBRb 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Comparative Electrode 102 200 Al — light-emitting Electron-injection layer 119 1 LiF — element 1 Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:PCzPCA1:Rubrene 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 20 DBT3P-II:MoO₃ 1:0.5 Electrode 101 110 ITSO — Comparative Electrode 102 200 Al — light-emitting Electron-injection layer 119 1 LiF — element 2 Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4mCzBPBfpm — Light-emitting layer 130 40 4mCzBPBfpm:PCzPCA1:Rubrene 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — <Fabrication of Light-emitting Element 1>

As the electrode 101, an ITSO film was formed to a thickness of 70 nm over a substrate. The area of the electrode 101 was set to 4 mm² (2 mm×2 mm).

Next, the EL layer 100 was formed over the electrode 101. As the hole-injection layer 111, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) and molybdenum oxide (MoO₃) were deposited by co-evaporation such that the deposited layer has a weight ratio of DBT3P-II to MoO₃ of 1:0.5 and a thickness of 60 nm. Then, as the hole-transport layer 112, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP) was deposited by evaporation to a thickness of 20 nm.

Next, as the light-emitting layer 130, 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm), 3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole (abbreviation: PCzPCA1), and 2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (abbreviation: TBRb) were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCzPCA1 and TBRb of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm and PCzPCA1 serve as the host material 131 and TBRb serves as the guest material 132 (fluorescent material).

As the electron-transport layer 118, 4,6mCzP2Pm and Bphen were sequentially deposited by evaporation to thicknesses of 20 nm and 10 nm, respectively, over the light-emitting layer 130. Next, as the electron-injection layer 119, lithium fluoride (LiF) was deposited by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was deposited to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, Light-emitting element 1 was sealed by fixing a sealing substrate to the substrate provided with the EL layer 100 using a sealant for an organic EL device. Specifically, after the sealant was applied to surround the EL layer 100 over the substrate and the substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm² and heat treatment at 80° C. for one hour were performed. Through the above process, Light-emitting element 1 was obtained.

<Fabrication of Light-emitting Element 2>

Light-emitting element 2 is different from the above-described Light-emitting element 1 in only the host material of the light-emitting layer 130 and the material of the electron-transport layer 118, and steps for the other components are the same as those in a method for fabricating Light-emitting element 1.

As the light-emitting layer 130 of Light-emitting element 2, 4-{3-[3′-(9H-carbazol-9-yl)]biphenyl-3-yl}benzofuro[3,2-d]pyrimidine (abbreviation: 4mCzBPBfpm), PCzPCA1, and TBRb were deposited by co-evaporation such that the deposited layer has a weight ratio of 4mCzBPBfpm to PCzPCA1 and TBRb of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4mCzBPBfpm and PCzPCA1 serve as the host material 131 and TBRb serves as the guest material 132 (fluorescent material).

As the electron-transport layer 118, 4mCzBPBfpm and Bphen were sequentially deposited by evaporation to thicknesses of 20 nm and 10 nm, respectively, over the light-emitting layer 130.

<Fabrication of Comparative Light-emitting Element 1>

Comparative light-emitting element 1 is different from the above-described Light-emitting element 1 in the thicknesses of the electrode 101 and the hole-injection layer 111 and the guest material of the light-emitting layer 130, and steps for the other components are the same as those in a method for fabricating Light-emitting element 1.

As the electrode 101 of Comparative light-emitting element 1, an ITSO film was formed to a thickness of 110 nm. The area of the electrode 101 was set to 4 mm² (2 mm×2 mm).

Next, as the hole-injection layer 111 over the electrode 101, DBT3P-II and MoO₃ were deposited by co-evaporation such that the deposited layer has a weight ratio of DBT3P-II to MoO₃ of 1:0.5 and a thickness of 20 nm.

As the light-emitting layer 130 of Comparative light-emitting element 1, 4,6mCzP2Pm, PCzPCA1, and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCzPCA1 and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm and PCzPCA1 serve as the host material 131 and Rubrene serves as the guest material 132 (fluorescent material).

<Fabrication of Comparative Light-emitting Element 2>

Comparative light-emitting element 2 is different from the above-described Light-emitting element 2 in only the guest material of the light-emitting layer 130, and steps for the other components are the same as those in a method for fabricating Light-emitting element 2.

As the light-emitting layer 130 of Comparative light-emitting element 2, 4mCzBPBfpm, PCzPCA1, and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4mCzBPBfpm to PCzPCA1 and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4mCzBPBfpm and PCzPCA1 serve as the host material 131 and Rubrene serves as the guest material 132 (fluorescent material).

<Operation Characteristics of Light-emitting Elements>

Next, emission characteristics of the fabricated Light-emitting elements 1 and 2 and Comparative light-emitting elements 1 and 2 were measured. Note that the measurement was performed at room temperature (in an atmosphere kept at 23° C.).

The emission characteristics of the light-emitting elements at a luminance around 1000 cd/m² are shown below in Table 3. The current efficiency-luminance characteristics, external quantum efficiency-luminance characteristics, and luminance-voltage characteristics of the light-emitting elements are shown in FIG. 33, FIG. 34, and FIG. 35, respectively. FIG. 36 shows electroluminescence spectra at the time when a current was made to flow in the light-emitting elements at a current density of 2.5 mA/cm².

TABLE 3 Current CIE Current Power External Voltage density chromaticity Luminance efficiency efficiency quantum (V) (mA/cm²) (x, y) (cd/m²) (cd/A) (lm/W) efficiency (%) Light-emitting 3.4 2.1 (0.48, 0.51) 1100 51 47 15 element 1 Light-emitting 3.0 1.8 (0.49, 0.51) 950 54 56 16 element 2 Comparative light- 3.9 3.9 (0.49, 0.50) 1100 28 23 8.6 emitting element 1 Comparative light- 3.5 4.1 (0.50, 0.50) 1100 27 24 8.2 emitting element 2

As shown by the peaks in the electroluminescence spectra in FIG. 36, only yellow light emissions derived from TBRb and Rubrene, which are fluorescent materials, were observed from Light-emitting elements 1 and 2 and Comparative light-emitting elements 1 and 2.

In addition, as shown in FIG. 33, FIG. 34, and Table 3, Light-emitting elements 1 and 2 have high current efficiency and high external quantum efficiency. Since the probability of formation of singlet excitons which are generated by recombination of carriers (holes and electrons) injected from the pair of electrodes is at most 25%, the external quantum efficiency in the case where the light extraction efficiency to the outside is 30% is at most 7.5%. Light-emitting elements 1 and 2 and Comparative light-emitting elements 1 and 2 can have external quantum efficiency of higher than 7.5%. This is because Light-emitting elements 1 and 2 and Comparative light-emitting elements 1 and 2 emit, in addition to light originating from singlet excitons generated by recombination of carriers (holes and electrons) injected from the pair of electrodes, light originating from singlet excitons generated from triplet excitons by ExEF.

In particular, Light-emitting elements 1 and 2 have external quantum efficiency of 15% or higher at a luminance around 1000 cd/m². The fluorescence quantum yield of Rubrene and the fluorescence quantum yield of TBRb which were measured by the measurement of the absolute fluorescence quantum yield are 61% and 90%, respectively; that is, the fluorescence quantum yield of TBRb is about 1.5 times as high as that of Rubrene. Furthermore, the external quantum efficiency of Light-emitting element 1 containing TBRb as a guest material is 1.7 times as high as that of Comparative light-emitting element 1 containing Rubrene as a guest material, and the external quantum efficiency of Light-emitting element 2 containing TBRb as a guest material is 1.9 times as high as that of Comparative light-emitting element 2 containing Rubrene as a guest material. That is, the external quantum efficiencies of Light-emitting elements 1 and 2 are increased by more than the difference between the fluorescence quantum yields of the guest materials.

As described in Embodiment 1, TBRb used as a guest material in Light-emitting elements 1 and 2 includes two or more tert-butyl groups; thus, the minimum value of the distance between the centroids of the guest material and the host material in the case where TBRb is used is larger than the case where Rubrene is used. Thus, the energy transfer from the host material to the guest material by Dexter mechanism is inhibited. The efficiency of energy transfer from the triplet excited state of the host material to the triplet excited state of the guest material was reduced and the generation efficiency of the singlet excited state in the light-emitting layer 130 was improved; therefore, it is concluded that Light-emitting elements 1 and 2 had high emission efficiency.

Light-emitting element 1 has high emission efficiency which is almost the same as that of Light-emitting element 2; thus, light-emitting elements having high emission efficiency can be fabricated when they contain the guest material described in Embodiment 1 even if the host materials are different.

Moreover, as shown in FIG. 35 and Table 3, Light-emitting elements 1 and 2 drive at a low voltage. That is, by including a light-emitting layer using ExEF, a light-emitting element that drives at a low voltage can be fabricated. Furthermore, a light-emitting element with reduced power consumption can be fabricated.

<Time-resolved Fluorescence Measurement of Light-emitting Elements>

Next, to examine whether Light-emitting elements 1 and 2 and Comparative light-emitting elements 1 and 2 emit light by ExEF, the fluorescence lifetimes were measured using the time-resolved fluorescence measurement.

A picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K.) was used for the measurement. To measure the lifetimes of fluorescence in the light-emitting elements, a square wave pulse voltage was applied to the light-emitting elements, and time-resolved measurement of light, which was attenuated from the falling of the voltage, was performed using a streak camera. The pulse voltage was applied at a frequency of 10 Hz. By integrating data obtained by repeated measurement, data with a high S/N ratio was obtained. The measurement was performed under the following conditions: the room temperature was 300 K; a pulse voltage of approximately from 3 V to 4 V was applied so that the luminance of the light-emitting element becomes around 1000 cd/m²; the pulse time width was 100 μsec; the negative bias voltage was −5V (when the element operation is OFF); and the measurement time was 50 μsec. The measurement results of the fluorescence lifetimes of the light-emitting elements are shown in FIGS. 37A and 37B. In FIGS. 37A and 37B, the vertical axis represents emission intensity normalized to that in a state where carriers are steadily injected (when the pulse voltage is ON), and the horizontal axis represents time elapsed after the falling of the pulse voltage.

The attenuation curves shown in FIGS. 37A and 37B were fitted with Formula (6).

$\begin{matrix} {L = {\sum\limits_{n = 1}{A_{n}{\exp\left( {- \frac{t}{a_{n}}} \right)}}}} & (6) \end{matrix}$

In Formula (6), L and t represent normalized emission intensity and elapsed time, respectively. The attenuation curve was able to be fitted when n was 1 to 3. This fitting results show that, when the proportion of a prompt fluorescence component and the fluorescence lifetime thereof were set to A₁ and a₁, respectively, and the proportion of a component whose lifetime is the shortest in the delayed fluorescence components and the fluorescence lifetime thereof were set to A₂ and a₂, respectively, the emission components of Light-emitting elements 1 and 2 and Comparative light-emitting elements 1 and 2 contain a prompt fluorescent component having a fluorescence lifetime of 0.3 μs and a delayed fluorescence component having a fluorescence lifetime of 3.0 μs. In addition, the percentages of the delayed fluorescence component in light emitted from Light-emitting element 1, Light-emitting element 2, Comparative light-emitting element 1, and Comparative light-emitting element 2 were calculated to 19%, 13%, 18%, and 10%, respectively. Therefore, it is found that Light-emitting element 1 contains the delayed fluorescence component at a higher percentage than that in Comparative light-emitting element 1, and Light-emitting element 2 contains the delayed fluorescence component at higher percentage than that in Comparative light-emitting element 2.

That is, when TBRb is used as a guest material, a light-emitting element containing a delayed fluorescence component at a high proportion and having high emission efficiency can be fabricated.

Furthermore, FIGS. 38A and 38B and FIGS. 39A and 39B show prompt fluorescence components (Prompt) and delayed fluorescence components (Delayed) of transient electroluminescence spectra in Light-emitting element 1, Light-emitting element 2, Comparative light-emitting element 1, and Comparative light-emitting element 2, which were measured using time-resolved fluorescence measurement, respectively. As shown in FIGS. 38A and 38B and FIGS. 39A and 39B, the emission spectra of the guest materials (TBRb and Rubrene) in the prompt fluorescence components substantially correspond to those in the delayed fluorescence components. Therefore, it indicates that the singlet excited energy of the singlet excitons generated by recombination of carriers injected from the pair of electrodes and the singlet excitons generated by reverse intersystem crossing in the exciplex is transferred to the singlet excited energy level of the guest materials (TBRb and Rubrene).

The structure described in this example can be combined as appropriate with any of the embodiments and the other examples.

EXAMPLE 2

In Example 2, examples of fabricating Light-emitting element 3 and Light-emitting element 4 of embodiments of the present invention will be described. In addition, examples of fabricating Comparative light-emitting element 3 and Comparative light-emitting element 4 will be described. Schematic cross-sectional views of light-emitting elements fabricated in Example 2 are similar to that of the light-emitting element 250 in FIG. 1A. Table 4 shows the detailed structures of the elements. In addition, structures and abbreviations of compounds used in Example 2 are given below. Note that Embodiment 1 and Example 1 can be referred to for other compounds.

TABLE 4 Film Reference thickness Layer numeral (nm) Material Weight ratio Light-emitting Electrode 102 200 Al — element 3 Electron-injection layer 119 1 LiF — Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:PCBiF:TBRb 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Light-emitting Electrode 102 200 Al — element 4 Electron-injection layer 119 1 LiF — Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:mPCBiF:TBRb 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Comparative Electrode 102 200 Al — light-emitting Electron-injection layer 119 1 LiF — element 3 Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:PCBiF:Rubrene 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 20 DBT3P-II:MoO₃ 1:0.5 Electrode 101 110 ITSO — Comparative Electrode 102 200 Al — light-emitting Electron-injection layer 119 1 LiF — element 4 Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4,6mCzP2Pm — Light-emitting layer 130 40 4,6mCzP2Pm:mPCBiF:Rubrene 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 20 DBT3P-II:MoO₃ 1:0.5 Electrode 101 110 ITSO — <Fabrication of Light-emitting Element 3>

Light-emitting element 3 is different from the above-described Light-emitting element 1 in only the host material of the light-emitting layer 130, and steps for the other components are the same as those in a method for fabricating Light-emitting element 1.

As the light-emitting layer 130 of Light-emitting element 3, 4,6mCzP2Pm, N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine (abbreviation: PCBiF), and TBRb were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCBiF and TBRb of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm and PCBiF serve as the host material 131 and TBRb serves as the guest material 132 (fluorescent material).

<Fabrication of Light-emitting Element 4>

Light-emitting element 4 is different from the above-described Light-emitting element 1 in only the host material of the light-emitting layer 130, and steps for the other components are the same as those in a method for fabricating Light-emitting element 1.

As the light-emitting layer 130 of Light-emitting element 4, 4,6mCzP2Pm, N-(3-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine (abbreviation: mPCBiF), and TBRb were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to mPCBiF and TBRb of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm and mPCBiF serve as the host material 131 and TBRb serves as the guest material 132 (fluorescent material).

<Fabrication of Comparative Light-emitting Element 3>

Comparative light-emitting element 3 is different from the above-described Comparative light-emitting element 1 in only the host material of the light-emitting layer 130, and steps for the other components are the same as those in a method for fabricating Comparative light-emitting element 1.

As the light-emitting layer 130 of Comparative light-emitting element 3, 4,6mCzP2Pm, PCBiF, and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to PCBiF and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm and PCBiF serve as the host material 131 and Rubrene serves as the guest material 132 (fluorescent material).

<Fabrication of Comparative Light-emitting Element 4>

Comparative light-emitting element 4 is different from the above-described Comparative light-emitting element 3 in only the structure of the light-emitting layer 130, and steps for the other components are the same as those in a method for fabricating Comparative light-emitting element 3. Note that the light-emitting layer 130 of Comparative light-emitting element 4 is different from the light-emitting layer 130 of Light-emitting element 4 in only the guest material.

As the light-emitting layer 130 of Comparative light-emitting element 4, 4,6mCzP2Pm, mPCBiF, and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4,6mCzP2Pm to mPCBiF and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4,6mCzP2Pm and mPCBiF serve as the host material 131 and Rubrene serves as the guest material 132 (fluorescent material).

<Operation Characteristics of Light-emitting Elements>

Next, emission characteristics of the fabricated Light-emitting elements 3 and 4 (Element 3 and Element 4) and Comparative light-emitting elements 3 and 4 (Comparative Element 3 and Comparative Element 4) were measured. Note that the measurement was performed at room temperature (in an atmosphere kept at 23° C.).

The emission characteristics of the light-emitting elements at a luminance around 1000 cd/m² are shown below in Table 5. The current efficiency-luminance characteristics, external quantum efficiency-luminance characteristics, and luminance-voltage characteristics of the light-emitting elements are shown in FIG. 40, FIG. 41, and FIG. 42, respectively. FIG. 43 shows electroluminescence spectra at the time when a current was made to flow in the light-emitting elements at a current density of 2.5 mA/cm².

TABLE 5 Current CIE Current Power External Voltage density chromaticity Luminance efficiency efficiency quantum (V) (mA/cm2) (x, y) (cd/m2) (cd/A) (lm/W) efficiency (%) Light-emitting 3.4 2.1 (0.49, 0.51) 1000 47 43 14 element 3 Light-emitting 3.4 1.9 (0.47, 0.52) 1030 53 49 16 element 4 Comparative light- 3.8 3.8 (0.48, 0.51) 1100 29 24 8.5 emitting element 3 Comparative light- 3.6 2.8 (0.47, 0.52) 900 32 28 9.3 emitting element 4

As shown by the peaks in the electroluminescence spectra in FIG. 43, only yellow light emissions derived from TBRb and Rubrene, which are fluorescent materials, were observed from Light-emitting elements 3 and 4 and Comparative light-emitting elements 3 and 4.

In addition, as shown in FIG. 40, FIG. 41, and Table 5, Light-emitting elements 3 and 4 have high current efficiency and high external quantum efficiency. Light-emitting elements 3 and 4 and Comparative light-emitting elements 3 and 4 can have external quantum efficiency of higher than 7.5%. This is because Light-emitting elements 3 and 4 and Comparative light-emitting elements 3 and 4 emit, in addition to light originating from singlet excitons generated by recombination of carriers (holes and electrons) injected from the pair of electrodes, light originating from singlet excitons generated from triplet excitons by ExEF.

In particular, Light-emitting elements 3 and 4 have external quantum efficiency of 14% or higher at a luminance around 1000 cd/m². Furthermore, the external quantum efficiency of Light-emitting element 3 is 1.6 times as high as that of Comparative light-emitting element 3, and the external quantum efficiency of Light-emitting element 4 is 1.7 times as high as that of Comparative light-emitting element 4. That is, the external quantum efficiencies of Light-emitting elements 3 and 4 are increased by more than the difference between the fluorescence quantum yields of the guest materials shown in Example 1.

As described in Embodiment 1, TBRb used as a guest material in Light-emitting elements 3 and 4 includes two or more tert-butyl groups; thus, the minimum value of the distance between the centroids of the guest material and the host material in the case where TBRb is used is larger than the case where Rubrene is used. Thus, the generation efficiency of the singlet excited state in the light-emitting layer 130 was improved, and it is concluded that Light-emitting elements 3 and 4 had high emission efficiency.

Light-emitting element 3 has high emission efficiency which is almost the same as that of Light-emitting element 4; thus, light-emitting elements having high emission efficiency can be fabricated when they contain the guest material described in Embodiment 1 even if the host materials are different.

Moreover, as shown in FIG. 42 and Table 5, Light-emitting elements 3 and 4 drive at a low voltage. That is, by including a light-emitting layer using ExEF, a light-emitting element that drives at a low voltage can be fabricated. Furthermore, a light-emitting element with reduced power consumption can be fabricated.

<Time-resolved Fluorescence Measurement of Light-emitting Elements>

Next, to examine whether Light-emitting elements 3 and 4 and Comparative light-emitting elements 3 and 4 emit light by ExEF, the fluorescence lifetimes were measured using the time-resolved fluorescence measurement.

Examples 1 may be referred to for the measurement method. The measurement results are shown in FIGS. 44A and 44B. The attenuation curves in FIGS. 44A and 44B were able to be fitted using Formula (6) when n was 1 to 3.

This fitting results show that, when the proportion of a prompt fluorescence component and the fluorescence lifetime thereof were set to A₁ and a₁, respectively, and the proportion of a component whose lifetime is the shortest in the delayed fluorescence components and the fluorescence lifetime thereof were set to A₂ and a₂, respectively, the emission components of Light-emitting elements 3 and 4 and Comparative light-emitting elements 3 and 4 contain a prompt fluorescent component having a fluorescence lifetime of 0.3 μs and a delayed fluorescence component having a fluorescence lifetime of 3.0 μs. In addition, the percentages of the delayed fluorescence component in light emitted from Light-emitting element 3, Light-emitting element 4, Comparative light-emitting element 3, and Comparative light-emitting element 4 were calculated to 38%, 38%, 28%, and 28%, respectively. Therefore, it is found that Light-emitting element 3 contains the delayed fluorescence component at a higher percentage than that in Comparative light-emitting element 3, and Light-emitting element 4 contains the delayed fluorescence component at higher percentage than that in Comparative light-emitting element 4.

That is, when TBRb is used as a guest material, a light-emitting element containing a delayed fluorescence component at a high proportion and having high emission efficiency can be fabricated.

Furthermore, FIGS. 45A and 45B and FIGS. 46A and 46B show prompt fluorescence components (Prompt) and delayed fluorescence components (Delayed) of transient electroluminescence spectra in Light-emitting element 3, Light-emitting element 4, Comparative light-emitting element 3, and Comparative light-emitting element 4, which were measured using time-resolved fluorescence measurement, respectively. As shown in FIGS. 45A and 45B and FIGS. 46A and 46B, the emission spectra of the guest materials (TBRb and Rubrene) in the prompt fluorescence components substantially correspond to those in the delayed fluorescence components. Therefore, it indicates that the singlet excited energy of the singlet excitons generated by recombination of carriers injected from the pair of electrodes and the singlet excitons generated by reverse intersystem crossing in the exciplex is transferred to the singlet excited energy level of the guest materials (TBRb and Rubrene).

The structure described in this example can be combined as appropriate with any of the embodiments and the other examples.

EXAMPLE 3

In Example 3, examples of fabricating Light-emitting element 5 (Element 5) and Comparative light-emitting element 5 (Comparative Element 5) of embodiments of the present invention will be described. Schematic cross-sectional views of light-emitting elements fabricated in Example 3 are similar to that of the light-emitting element 250 in FIG. 1A. Table 6 shows the detailed structures of the elements. In addition, structures and abbreviations of compounds used in Example 3 are given below. Note that Embodiment 1 and Example 1 can be referred to for other compounds.

TABLE 6 Film Reference thickness Layer numeral (nm) Material Weight ratio Light-emitting Electrode 102 200 Al — element 5 Electron-injection layer 119 1 LiF — Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4mCzBPBfpm — Light-emitting layer 130 40 4mCzBPBfpm:PCASF:TBRb 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Comparative Electrode 102 200 Al — light emitting Electron-injection layer 119 1 LiF — element 5 Electron-transport layer  118(2) 10 BPhen —  118(1) 20 4mCzBPBfpm — Light-emitting layer 130 40 4mCzBPBfpm:PCASF:Rubrene 0.8:0.2:0.01 Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — <Fabrication of Light-emitting Element 5>

Light-emitting element 5 is different from the above-described Light-emitting element 2 in only the host material of the light-emitting layer 130, and steps for the other components are the same as those in a method for fabricating Light-emitting element 2.

As the light-emitting layer 130 of Light-emitting element 5, 4mCzBPBfpm, 2-[N-(9-phenylcarbazol-3-yl)-N-phenyl amino]spiro-9,9′-bifluorene (abbreviation: PCASF), and TBRb were deposited by co-evaporation such that the deposited layer has a weight ratio of 4mCzBPBfpm to PCASF and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4mCzBPBfpm and PCASF serve as the host material 131 and TBRb serves as the guest material 132 (fluorescent material).

<Fabrication of Comparative Light-emitting Element 5>

Comparative light-emitting element 5 is different from the above-described Light-emitting element 5 in only the guest material of the light-emitting layer 130, and steps for the other components are the same as those in a method for fabricating Light-emitting element 5.

As the light-emitting layer 130 of Comparative light-emitting element 5, 4mCzBPBfpm, PCASF, and Rubrene were deposited by co-evaporation such that the deposited layer has a weight ratio of 4mCzBPBfpm to PCASF and Rubrene of 0.8:0.2:0.01 and a thickness of 40 nm. In the light-emitting layer 130, 4mCzBPBfpm and PCASF serve as the host material 131 and Rubrene serves as the guest material 132 (fluorescent material).

<Operation Characteristics of Light-emitting Elements>

Next, emission characteristics of the fabricated Light-emitting element 5 and Comparative light-emitting element 5 were measured. Note that the measurement was performed at room temperature (in an atmosphere kept at 23° C.).

The emission characteristics of the light-emitting elements at a luminance around 1000 cd/m² are shown below in Table 7. The current efficiency-luminance characteristics, external quantum efficiency-luminance characteristics, and luminance-voltage characteristics of the light-emitting elements are shown in FIG. 47, FIG. 48, and FIG. 49, respectively. FIG. 50 shows electroluminescence spectra at the time when a current was made to flow in the light-emitting elements at a current density of 2.5 mA/cm².

TABLE 7 Current CIE Current Power External Voltage density chromaticity Luminance efficiency efficiency quantum (V) (mA/cm2) (x, y) (cd/m2) (cd/A) (lm/W) efficiency (%) Light-emitting 3.0 1.9 (0.48, 0.51) 1010 53 56 16 element 5 Comparative light- 3.4 3.2 (0.48, 0.51) 1010 32 30 9.6 emitting element 5

As shown by the peaks in the electroluminescence spectra in FIG. 50, only yellow light emissions derived from TBRb and Rubrene, which are fluorescent materials, were observed from Light-emitting element 5 and Comparative light-emitting element 5.

In addition, as shown in FIG. 47, FIG. 48, and Table 7, Light-emitting element 5 has high current efficiency and high external quantum efficiency. Light-emitting element 5 and Comparative light-emitting element 5 can have external quantum efficiency of higher than 7.5%. This is because Light-emitting element 5 and Comparative light-emitting element 5 emit, in addition to light originating from singlet excitons generated by recombination of carriers (holes and electrons) injected from the pair of electrodes, light originating from singlet excitons generated from triplet excitons by ExEF.

In particular, Light-emitting element 5 has external quantum efficiency of 14% or higher at a luminance around 1000 cd/m². Furthermore, the external quantum efficiency of Light-emitting element 5 is 1.7 times as high as that of Comparative light-emitting element 5. That is, the external quantum efficiency of Light-emitting element 5 is increased by more than the difference between the fluorescence quantum yields of the guest materials shown in Example 1.

As described in Embodiment 1, TBRb used as a guest material in Light-emitting element 5 includes two or more tert-butyl groups; thus, the minimum value of the distance between the centroids of the guest material and the host material in the case where TBRb is used is larger than the case where Rubrene is used. Thus, the generation efficiency of the singlet excited state in the light-emitting layer 130 was improved, and it is concluded that Light-emitting element 5 had high emission efficiency.

Moreover, as shown in FIG. 49 and Table 7, Light-emitting element 5 drives at a low voltage. That is, by including a light-emitting layer using ExEF, a light-emitting element that drives at a low voltage can be fabricated. Furthermore, a light-emitting element with reduced power consumption can be fabricated.

The structure described in this example can be combined as appropriate with any of the embodiments and the other examples.

EXAMPLE 4

Example 4 shows time-resolved fluorescence measurement results of light-emitting layers of light-emitting elements which are embodiments of the present invention. Structures and abbreviations of compounds used in Example 4 are given below. Embodiment 1 may be referred to for other compounds.

<Fabrication of Thin-film Samples>

For the time-resolved fluorescence measurement of light-emitting layers of light-emitting elements, thin-film samples were fabricated on a quartz substrate by a vacuum evaporation method.

As Thin film 1, 4,6mCzP2Pm, N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine (abbreviation: PCBBiF), and TBRb were deposited by co-evaporation such that the deposited film has a weight ratio of 4,6mCzP2Pm to PCBBiF and Rubrene of 0.8:0.2:0.01 and a thickness of 50 nm. In Thin film 1, 4,6mCzP2Pm and PCBBiF serve as the host material 131 and TBRb serves as the guest material 132 (fluorescent material).

As Thin film 2, 4,6mCzP2Pm, PCBBiF, and Rubrene were deposited by co-evaporation such that the deposited film has a weight ratio of 4,6mCzP2Pm to PCBBiF and Rubrene of 0.8:0.2:0.01 and a thickness of 50 nm. In Thin film 2, 4,6mCzP2Pm and PCBBiF serve as the host material 131 and Rubrene serves as the guest material 132 (fluorescent material).

As Thin film 3, 4,6mCzP2Pm and PCBBiF were deposited by co-evaporation such that the deposited film has a weight ratio of 4,6mCzP2Pm to PCBBiF of 0.8:0.2 and a thickness of 50 nm. In Thin film 3, 4,6mCzP2Pm and PCBBiF correspond to the host material 131 and the guest material 132 (fluorescent material) is not contained.

As Thin film 4, 4,6mCzP2Pm and PCASF were deposited by co-evaporation such that the deposited film has a weight ratio of 4,6mCzP2Pm to PCASF of 0.8:0.2 and a thickness of 50 nm. In Thin film 4, 4,6mCzP2Pm and PCASF correspond to the host material 131 and the guest material 132 (fluorescent material) is not contained.

As Thin film 5, 4mCzBPBfpm and PCBBiF were deposited by co-evaporation such that the deposited film has a weight ratio of 4mCzBPBfpm to PCBBiF of 0.8:0.2 and a thickness of 50 nm. In Thin film 5, 4mCzBPBfpm and PCBBiF correspond to the host material 131 and the guest material 132 (fluorescent material) is not contained.

As Thin film 6, 4mCzBPBfpm and PCASF were deposited by co-evaporation such that the deposited film has a weight ratio of 4mCzBPBfpm to PCASF of 0.8:0.2 and a thickness of 50 nm. In Thin film 6, 4mCzBPBfpm and PCASF correspond to the host material 131 and the guest material 132 (fluorescent material) is not contained.

Thin films 1 to 6 were each sealed by fixing a sealing substrate to the quartz substrate over which the thin-film sample was deposited using a sealant for an organic EL device in a glove box under a nitrogen atmosphere. Specifically, after a sealant was applied to surround each of the thin films over the quartz substrate and the quartz substrate was bonded to the sealing substrate, irradiation with ultraviolet light having a wavelength of 365 nm at 6 J/cm² and heat treatment at 80° C. for one hour were performed.

<Time-resolved Fluorescence Measurement of Thin-film Samples>

A picosecond fluorescence lifetime measurement system (manufactured by Hamamatsu Photonics K.K.) was used for the measurement. In this measurement, the thin film was irradiated with pulsed laser, and emission of the thin film which was attenuated from the laser irradiation underwent time-resolved measurement using a streak camera to measure the lifetime of fluorescent emission of the thin film. A nitrogen gas laser with a wavelength of 337 nm was used as the pulsed laser. The thin film was irradiated with pulsed laser with a pulse width of 500 ps at a repetition rate of 10 Hz. By integrating data obtained by the repeated measurement, data with a high S/N ratio was obtained. The measurement was performed at room temperature (in an atmosphere kept at 23° C.). The measurement range was 1 ms.

From Thin films 1 and 2, yellow light emission derived from TBRb and Rubrene, which are guest materials, were observed. In addition, from Thin film 3, an emission spectrum whose maximum value is around 510 nm of an exciplex formed of 4,6mCzP2Pm and PCBBiF was obtained. From Thin film 4, an emission spectrum whose maximum value is around 545 nm of an exciplex formed of 4,6mCzP2Pm and PCASF was obtained. From Thin film 5, an emission spectrum whose maximum value is around 549 nm of an exciplex formed of 4mCzBPBfpm and PCBBiF was obtained. From Thin film 6, an emission spectrum whose maximum value is around 551 nm of an exciplex formed of 4mCzBPBfpm and PCASF was obtained. The attenuation curves obtained by the measurement are shown in FIG. 51 and FIGS. 52A and 52B.

As shown in FIG. 51, the attenuation curves of Thin films 1 and 2 substantially correspond to each other. In this example, the host material is excited by photoexitation, lights emitted from the guest materials contained in Thin films 1 and 2 are observed. That is, with the conditions, only a singlet excited state is generated in Thin films 1 and 2. Since the fluorescence lifetimes of Thin films 1 and 2 substantially correspond to each other, it indicates that there is little difference between the energy transfers from the singlet excited state of the host material to the singlet excited state of the guest material even if the guest materials are different.

When the guest materials described in Embodiment 1 are used in the light-emitting elements in Examples 1 to 3, the proportion of the delayed fluorescence components is increased and the emission efficiency is improved. It is concluded that this is because the energy transfer from the triplet excited state of the host material to the triplet excited state of the guest material is inhibited and the generation efficiency of the singlet excitons is improved owing to reverse intersystem crossing in the exciplex.

The attenuation curves of Thin films 3 to 6 in FIGS. 52A and 52B were able to be fitted using Formula (6) when n was 1 to 3. This fitting results show that the emission components of Thin film 3, Thin film 4, Thin film 5, and Thin film 6 contain, in addition to an early fluorescent component having a fluorescence lifetime of 3.9 μs, delayed fluorescence components having fluorescence lifetimes of 58 μs, 30 μs, 27 μs, and 16 μs, respectively. In addition, the percentages of the delayed fluorescence component in light emitted from Thin film 3, Thin film 4, Thin film 5, and Thin film 6 were calculated to 10%, 24%, 33%, and 31%, respectively.

An exciplex has a feature of having a singlet excited energy level and a triplet excited energy level that are close to each other. The delayed fluorescence observed in Thin films 3 to 6 is caused by thermally activated delayed fluorescence due to intersystem crossing and reverse intersystem crossing between the singlet excited state and the triplet excited state of the exciplex. Thus, Thin films 3 to 6 are exciplexes that are suitable for embodiments of the present invention. Furthermore, the delayed fluorescence observed in Thin films 4 to 6 includes the delayed fluorescent components of 50 us or less, which is relatively early; thus, it indicated that the rate constants of reverse intersystem crossing in these exciplexes are relatively large. Thus, the exciplexes in Thin films 4 to 6 are host materials suitable for a medium for energy transfer to guest materials.

The structure described in this example can be combined as appropriate with any of the embodiments and the other examples.

This application is based on Japanese Patent Application serial no. 2015-046409 filed with Japan Patent Office on Mar. 9, 2015, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A light-emitting element comprising: a pair of electrodes; and a light-emitting layer between the pair of electrodes, wherein the light-emitting layer comprises: a fluorescent material as a guest material; and a first organic compound and a second organic compound capable of forming an exciplex as a host material, and wherein a minimum value of a distance between a centroid of the fluorescent material and a centroid of the first organic compound or the second organic compound is larger than or equal to 0.7 nm and smaller than or equal to 5 nm; wherein the minimum value of the distance between the centroids is a distance at the time when a running coordination number calculated by a classical molecular dynamics method exceeds
 0. 2. The light-emitting element according to claim 1, wherein the minimum value of the distance between the centroids corresponds to a distance between the centroid of the fluorescent material and the centroid of the first organic compound or the second organic compound nearest to the fluorescent material.
 3. The light-emitting element according to claim 1, wherein a minimum value of a distance between the centroid of the fluorescent material and a centroid of the first organic compound and a minimum value of a distance between the centroid of the fluorescent material and a centroid of the second organic compound are each larger than or equal to 0.7 nm and smaller than or equal to 5 nm.
 4. The light-emitting element according to claim 1, wherein the fluorescent material comprises at least two alkyl groups each having 2 or more carbon atoms.
 5. The light-emitting element according to claim 1, wherein the fluorescent material comprises at least two branched alkyl groups each having 3 to 10 carbon atoms.
 6. The light-emitting element according to claim 1, wherein the fluorescent material comprises at least two cyclic hydrocarbon groups each having 3 to 10 carbon atoms or at least two bridged cyclic hydrocarbon groups each having 3 to 10 carbon atoms.
 7. The light-emitting element according to claim 1, wherein the fluorescent material comprises condensed aromatic hydrocarbon having 3 to 12 carbon atoms.
 8. The light-emitting element according to claim 1, wherein light emitted from the exciplex has a region overlapping with an absorption band of the fluorescent material on the lowest energy side.
 9. The light-emitting element according to claim 1, wherein a proportion of a delayed fluorescence component in light emitted from the exciplex is higher than or equal to 10%, and wherein the delayed fluorescence component contains a delayed fluorescence component whose fluorescence lifetime is 10 ns or longer and 50 μs or shorter.
 10. The light-emitting element according to claim 1, wherein one of the first organic compound and the second organic compound has a function of transporting electrons, and wherein the other of the first organic compound and the second organic compound has a function of transporting holes.
 11. The light-emitting element according to claim 1, wherein one of the first organic compound and the second organic compound comprises a π-electron deficient heteroaromatic ring skeleton, and wherein the other of the first organic compound and the second organic compound comprises a π-electron rich heteroaromatic ring skeleton or an aromatic amine skeleton.
 12. A display device comprising: the light-emitting element according to claim 1; and at least one of a color filter and a transistor.
 13. An electronic device comprising: the display device according to claim 12; and at least one of a housing and a touch sensor.
 14. A lighting device comprising: the light-emitting element according to claim 1; and at least one of a housing and a touch sensor.
 15. A light-emitting element comprising: a pair of electrodes; and a light-emitting layer between the pair of electrodes, wherein the light-emitting layer comprises: a fluorescent material as a guest material; and a first organic compound and a second organic compound capable of forming an exciplex as a host material, wherein the fluorescent material has a fluorescence quantum yield of 70% or higher, and wherein a minimum value of a distance between a centroid of the fluorescent material and a centroid of the first organic compound or the second organic compound is larger than or equal to 0.7 nm and smaller than or equal to 5 nm; wherein the minimum value of the distance between the centroids is a distance at the time when a running coordination number calculated by a classical molecular dynamics method exceeds
 0. 16. The light-emitting element according to claim 15, wherein the minimum value of the distance between the centroids corresponds to a distance between the centroid of the fluorescent material and the centroid of the first organic compound or the second organic compound nearest to the fluorescent material.
 17. The light-emitting element according to claim 15, wherein a minimum value of a distance between the centroid of the fluorescent material and a centroid of the first organic compound and a minimum value of a distance between the centroid of the fluorescent material and a centroid of the second organic compound are each larger than or equal to 0.7 nm and smaller than or equal to 5 nm.
 18. The light-emitting element according to claim 15, wherein the fluorescent material comprises at least two alkyl groups each having 2 or more carbon atoms.
 19. The light-emitting element according to claim 15, wherein the fluorescent material comprises at least two branched alkyl groups each having 3 to 10 carbon atoms.
 20. The light-emitting element according to claim 15, wherein the fluorescent material comprises at least two cyclic hydrocarbon groups each having 3 to 10 carbon atoms or at least two bridged cyclic hydrocarbon groups each having 3 to 10 carbon atoms.
 21. The light-emitting element according to claim 15, wherein the fluorescent material comprises condensed aromatic hydrocarbon having 3 to 12 carbon atoms.
 22. The light-emitting element according to claim 15, wherein light emitted from the exciplex has a region overlapping with an absorption band of the fluorescent material on the lowest energy side.
 23. The light-emitting element according to claim 15, wherein a proportion of a delayed fluorescence component in light emitted from the exciplex is higher than or equal to 10%, and wherein the delayed fluorescence component contains a delayed fluorescence component whose fluorescence lifetime is 10 ns or longer and 50 μs or shorter.
 24. The light-emitting element according to claim 15, wherein one of the first organic compound and the second organic compound has a function of transporting electrons, and wherein the other of the first organic compound and the second organic compound has a function of transporting holes.
 25. The light-emitting element according to claim 15, wherein one of the first organic compound and the second organic compound comprises a π-electron deficient heteroaromatic ring skeleton, and wherein the other of the first organic compound and the second organic compound comprises a π-electron rich heteroaromatic ring skeleton or an aromatic amine skeleton.
 26. A display device comprising: the light-emitting element according to claim 15; and at least one of a color filter and a transistor.
 27. An electronic device comprising: the display device according to claim 26; and at least one of a housing and a touch sensor.
 28. A lighting device comprising: the light-emitting element according to claim 15; and at least one of a housing and a touch sensor. 